This also just happened to me. The display is blank (the power led is blinking to signify that DPMS is off), the mouse and keyboard do not respond (the num lock led is stuck). Syslog has:

Dec 21 15:31:32 dov kernel: [81840.640063] INFO: task i915/0:360 blocked for more than 120 seconds.
Dec 21 15:31:32 dov kernel: [81840.640067] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:31:32 dov kernel: [81840.640070] i915/0        D 0000000000000000     0   360      2 0x00000000
Dec 21 15:31:32 dov kernel: [81840.640075]  ffff880122dedd70 0000000000000046 0000000000000000 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640079]  ffff8801234e47c0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640083]  0000000000015880 ffff8801234e47c0 0000000000015880 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640087] Call Trace:
Dec 21 15:31:32 dov kernel: [81840.640096]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:31:32 dov kernel: [81840.640100]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:31:32 dov kernel: [81840.640120]  [<ffffffffa00858d8>] i915_gem_retire_work_handler+0x38/0x90 [i915]
Dec 21 15:31:32 dov kernel: [81840.640130]  [<ffffffffa00858a0>] ? i915_gem_retire_work_handler+0x0/0x90 [i915]
Dec 21 15:31:32 dov kernel: [81840.640135]  [<ffffffff810737f5>] run_workqueue+0x95/0x170
Dec 21 15:31:32 dov kernel: [81840.640139]  [<ffffffff81073974>] worker_thread+0xa4/0x120
Dec 21 15:31:32 dov kernel: [81840.640142]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:31:32 dov kernel: [81840.640146]  [<ffffffff810738d0>] ? worker_thread+0x0/0x120
Dec 21 15:31:32 dov kernel: [81840.640149]  [<ffffffff81078736>] kthread+0xa6/0xb0
Dec 21 15:31:32 dov kernel: [81840.640153]  [<ffffffff810130ea>] child_rip+0xa/0x20
Dec 21 15:31:32 dov kernel: [81840.640156]  [<ffffffff81078690>] ? kthread+0x0/0xb0
Dec 21 15:31:32 dov kernel: [81840.640159]  [<ffffffff810130e0>] ? child_rip+0x0/0x20
Dec 21 15:31:32 dov kernel: [81840.640171] INFO: task Xorg:1299 blocked for more than 120 seconds.
Dec 21 15:31:32 dov kernel: [81840.640173] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:31:32 dov kernel: [81840.640175] Xorg          D 0000000000000000     0  1299   1253 0x00400000
Dec 21 15:31:32 dov kernel: [81840.640179]  ffff8800ce5ebcb8 0000000000000086 ffff8800ce5ebc58 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640183]  ffff8800ce5383b0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640187]  0000000000015880 ffff8800ce5383b0 0000000000015880 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640196] Call Trace:
Dec 21 15:31:32 dov kernel: [81840.640199]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:31:32 dov kernel: [81840.640201]  [<ffffffff81528739>] ? mutex_lock+0x19/0x50
Dec 21 15:31:32 dov kernel: [81840.640204]  [<ffffffff81528739>] ? mutex_lock+0x19/0x50
Dec 21 15:31:32 dov kernel: [81840.640206]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:31:32 dov kernel: [81840.640214]  [<ffffffffa0085bc6>] i915_gem_throttle_ioctl+0x36/0x90 [i915]
Dec 21 15:31:32 dov kernel: [81840.640227]  [<ffffffffa004bcae>] drm_ioctl+0x17e/0x3a0 [drm]
Dec 21 15:31:32 dov kernel: [81840.640231]  [<ffffffff81011545>] ? __setup_rt_frame+0x2c5/0x420
Dec 21 15:31:32 dov kernel: [81840.640242]  [<ffffffffa00560b0>] ? drm_mode_connector_property_set_ioctl+0x0/0x1c0 [drm]
Dec 21 15:31:32 dov kernel: [81840.640246]  [<ffffffff81036419>] ? default_spin_lock_flags+0x9/0x10
Dec 21 15:31:32 dov kernel: [81840.640249]  [<ffffffff81529b0a>] ? _spin_lock_irqsave+0x2a/0x40
Dec 21 15:31:32 dov kernel: [81840.640252]  [<ffffffff8112e49c>] vfs_ioctl+0x7c/0xa0
Dec 21 15:31:32 dov kernel: [81840.640254]  [<ffffffff8112ea69>] do_vfs_ioctl+0x79/0x370
Dec 21 15:31:32 dov kernel: [81840.640256]  [<ffffffff8112ede1>] sys_ioctl+0x81/0xa0
Dec 21 15:31:32 dov kernel: [81840.640259]  [<ffffffff81012002>] system_call_fastpath+0x16/0x1b
Dec 21 15:31:32 dov kernel: [81840.640264] INFO: task compiz.real:2110 blocked for more than 120 seconds.
Dec 21 15:31:32 dov kernel: [81840.640266] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:31:32 dov kernel: [81840.640267] compiz.real   D 0000000000000000     0  2110   2001 0x00000000
Dec 21 15:31:32 dov kernel: [81840.640270]  ffff88003786bca8 0000000000000082 ffff880000000040 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640274]  ffff8801000c83b0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640277]  0000000000015880 ffff8801000c83b0 0000000000015880 0000000000015880
Dec 21 15:31:32 dov kernel: [81840.640279] Call Trace:
Dec 21 15:31:32 dov kernel: [81840.640282]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:31:32 dov kernel: [81840.640285]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:31:32 dov kernel: [81840.640293]  [<ffffffffa0086c72>] i915_gem_set_domain_ioctl+0xa2/0x140 [i915]
Dec 21 15:31:32 dov kernel: [81840.640302]  [<ffffffffa004bcae>] drm_ioctl+0x17e/0x3a0 [drm]
Dec 21 15:31:32 dov kernel: [81840.640310]  [<ffffffffa0086bd0>] ? i915_gem_set_domain_ioctl+0x0/0x140 [i915]
Dec 21 15:31:32 dov kernel: [81840.640313]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:31:32 dov kernel: [81840.640316]  [<ffffffff8112e49c>] vfs_ioctl+0x7c/0xa0
Dec 21 15:31:32 dov kernel: [81840.640318]  [<ffffffff8112ea69>] do_vfs_ioctl+0x79/0x370
Dec 21 15:31:32 dov kernel: [81840.640321]  [<ffffffff8111f8b1>] ? vfs_read+0x181/0x1a0
Dec 21 15:31:32 dov kernel: [81840.640323]  [<ffffffff8112ede1>] sys_ioctl+0x81/0xa0
Dec 21 15:31:32 dov kernel: [81840.640325]  [<ffffffff81130747>] ? sys_poll+0x77/0x110
Dec 21 15:31:32 dov kernel: [81840.640328]  [<ffffffff81012002>] system_call_fastpath+0x16/0x1b
Dec 21 15:33:32 dov kernel: [81960.641314] INFO: task i915/0:360 blocked for more than 120 seconds.
Dec 21 15:33:32 dov kernel: [81960.641318] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:33:32 dov kernel: [81960.641321] i915/0        D 0000000000000000     0   360      2 0x00000000
Dec 21 15:33:32 dov kernel: [81960.641326]  ffff880122dedd70 0000000000000046 0000000000000000 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641330]  ffff8801234e47c0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641334]  0000000000015880 ffff8801234e47c0 0000000000015880 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641338] Call Trace:
Dec 21 15:33:32 dov kernel: [81960.641347]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:33:32 dov kernel: [81960.641351]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:33:32 dov kernel: [81960.641371]  [<ffffffffa00858d8>] i915_gem_retire_work_handler+0x38/0x90 [i915]
Dec 21 15:33:32 dov kernel: [81960.641382]  [<ffffffffa00858a0>] ? i915_gem_retire_work_handler+0x0/0x90 [i915]
Dec 21 15:33:32 dov kernel: [81960.641387]  [<ffffffff810737f5>] run_workqueue+0x95/0x170
Dec 21 15:33:32 dov kernel: [81960.641390]  [<ffffffff81073974>] worker_thread+0xa4/0x120
Dec 21 15:33:32 dov kernel: [81960.641394]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:33:32 dov kernel: [81960.641398]  [<ffffffff810738d0>] ? worker_thread+0x0/0x120
Dec 21 15:33:32 dov kernel: [81960.641401]  [<ffffffff81078736>] kthread+0xa6/0xb0
Dec 21 15:33:32 dov kernel: [81960.641404]  [<ffffffff810130ea>] child_rip+0xa/0x20
Dec 21 15:33:32 dov kernel: [81960.641408]  [<ffffffff81078690>] ? kthread+0x0/0xb0
Dec 21 15:33:32 dov kernel: [81960.641410]  [<ffffffff810130e0>] ? child_rip+0x0/0x20
Dec 21 15:33:32 dov kernel: [81960.641423] INFO: task Xorg:1299 blocked for more than 120 seconds.
Dec 21 15:33:32 dov kernel: [81960.641425] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:33:32 dov kernel: [81960.641427] Xorg          D 0000000000000000     0  1299   1253 0x00400000
Dec 21 15:33:32 dov kernel: [81960.641431]  ffff8800ce5ebcb8 0000000000000086 ffff8800ce5ebc58 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641435]  ffff8800ce5383b0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641439]  0000000000015880 ffff8800ce5383b0 0000000000015880 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641443] Call Trace:
Dec 21 15:33:32 dov kernel: [81960.641446]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:33:32 dov kernel: [81960.641450]  [<ffffffff81528739>] ? mutex_lock+0x19/0x50
Dec 21 15:33:32 dov kernel: [81960.641453]  [<ffffffff81528739>] ? mutex_lock+0x19/0x50
Dec 21 15:33:32 dov kernel: [81960.641456]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:33:32 dov kernel: [81960.641470]  [<ffffffffa0085bc6>] i915_gem_throttle_ioctl+0x36/0x90 [i915]
Dec 21 15:33:32 dov kernel: [81960.641483]  [<ffffffffa004bcae>] drm_ioctl+0x17e/0x3a0 [drm]
Dec 21 15:33:32 dov kernel: [81960.641487]  [<ffffffff81011545>] ? __setup_rt_frame+0x2c5/0x420
Dec 21 15:33:32 dov kernel: [81960.641498]  [<ffffffffa00560b0>] ? drm_mode_connector_property_set_ioctl+0x0/0x1c0 [drm]
Dec 21 15:33:32 dov kernel: [81960.641502]  [<ffffffff81036419>] ? default_spin_lock_flags+0x9/0x10
Dec 21 15:33:32 dov kernel: [81960.641505]  [<ffffffff81529b0a>] ? _spin_lock_irqsave+0x2a/0x40
Dec 21 15:33:32 dov kernel: [81960.641508]  [<ffffffff8112e49c>] vfs_ioctl+0x7c/0xa0
Dec 21 15:33:32 dov kernel: [81960.641510]  [<ffffffff8112ea69>] do_vfs_ioctl+0x79/0x370
Dec 21 15:33:32 dov kernel: [81960.641512]  [<ffffffff8112ede1>] sys_ioctl+0x81/0xa0
Dec 21 15:33:32 dov kernel: [81960.641515]  [<ffffffff81012002>] system_call_fastpath+0x16/0x1b
Dec 21 15:33:32 dov kernel: [81960.641520] INFO: task compiz.real:2110 blocked for more than 120 seconds.
Dec 21 15:33:32 dov kernel: [81960.641522] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:33:32 dov kernel: [81960.641524] compiz.real   D 0000000000000000     0  2110   2001 0x00000000
Dec 21 15:33:32 dov kernel: [81960.641527]  ffff88003786bca8 0000000000000082 ffff880000000040 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641530]  ffff8801000c83b0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641533]  0000000000015880 ffff8801000c83b0 0000000000015880 0000000000015880
Dec 21 15:33:32 dov kernel: [81960.641536] Call Trace:
Dec 21 15:33:32 dov kernel: [81960.641538]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:33:32 dov kernel: [81960.641541]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:33:32 dov kernel: [81960.641549]  [<ffffffffa0086c72>] i915_gem_set_domain_ioctl+0xa2/0x140 [i915]
Dec 21 15:33:32 dov kernel: [81960.641559]  [<ffffffffa004bcae>] drm_ioctl+0x17e/0x3a0 [drm]
Dec 21 15:33:32 dov kernel: [81960.641566]  [<ffffffffa0086bd0>] ? i915_gem_set_domain_ioctl+0x0/0x140 [i915]
Dec 21 15:33:32 dov kernel: [81960.641569]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:33:32 dov kernel: [81960.641572]  [<ffffffff8112e49c>] vfs_ioctl+0x7c/0xa0
Dec 21 15:33:32 dov kernel: [81960.641574]  [<ffffffff8112ea69>] do_vfs_ioctl+0x79/0x370
Dec 21 15:33:32 dov kernel: [81960.641577]  [<ffffffff8111f8b1>] ? vfs_read+0x181/0x1a0
Dec 21 15:33:32 dov kernel: [81960.641579]  [<ffffffff8112ede1>] sys_ioctl+0x81/0xa0
Dec 21 15:33:32 dov kernel: [81960.641581]  [<ffffffff81130747>] ? sys_poll+0x77/0x110
Dec 21 15:33:32 dov kernel: [81960.641584]  [<ffffffff81012002>] system_call_fastpath+0x16/0x1b
Dec 21 15:35:01 dov CRON[23309]: (root) CMD (if [ -x /etc/munin/plugins/apt_all ]; then /etc/munin/plugins/apt_all update 7200 12 >/dev/null; elif [ -x /etc/munin/plugins/apt ]; then /etc/munin/plugins/apt update 7200 12 >/dev/null; fi)
Dec 21 15:35:32 dov kernel: [82080.641315] INFO: task i915/0:360 blocked for more than 120 seconds.
Dec 21 15:35:32 dov kernel: [82080.641318] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:35:32 dov kernel: [82080.641321] i915/0        D 0000000000000000     0   360      2 0x00000000
Dec 21 15:35:32 dov kernel: [82080.641326]  ffff880122dedd70 0000000000000046 0000000000000000 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641330]  ffff8801234e47c0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641334]  0000000000015880 ffff8801234e47c0 0000000000015880 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641338] Call Trace:
Dec 21 15:35:32 dov kernel: [82080.641347]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:35:32 dov kernel: [82080.641351]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:35:32 dov kernel: [82080.641370]  [<ffffffffa00858d8>] i915_gem_retire_work_handler+0x38/0x90 [i915]
Dec 21 15:35:32 dov kernel: [82080.641381]  [<ffffffffa00858a0>] ? i915_gem_retire_work_handler+0x0/0x90 [i915]
Dec 21 15:35:32 dov kernel: [82080.641386]  [<ffffffff810737f5>] run_workqueue+0x95/0x170
Dec 21 15:35:32 dov kernel: [82080.641390]  [<ffffffff81073974>] worker_thread+0xa4/0x120
Dec 21 15:35:32 dov kernel: [82080.641393]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:35:32 dov kernel: [82080.641397]  [<ffffffff810738d0>] ? worker_thread+0x0/0x120
Dec 21 15:35:32 dov kernel: [82080.641400]  [<ffffffff81078736>] kthread+0xa6/0xb0
Dec 21 15:35:32 dov kernel: [82080.641404]  [<ffffffff810130ea>] child_rip+0xa/0x20
Dec 21 15:35:32 dov kernel: [82080.641407]  [<ffffffff81078690>] ? kthread+0x0/0xb0
Dec 21 15:35:32 dov kernel: [82080.641410]  [<ffffffff810130e0>] ? child_rip+0x0/0x20
Dec 21 15:35:32 dov kernel: [82080.641422] INFO: task Xorg:1299 blocked for more than 120 seconds.
Dec 21 15:35:32 dov kernel: [82080.641424] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:35:32 dov kernel: [82080.641426] Xorg          D 0000000000000000     0  1299   1253 0x00400000
Dec 21 15:35:32 dov kernel: [82080.641430]  ffff8800ce5ebcb8 0000000000000086 ffff8800ce5ebc58 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641434]  ffff8800ce5383b0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641438]  0000000000015880 ffff8800ce5383b0 0000000000015880 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641442] Call Trace:
Dec 21 15:35:32 dov kernel: [82080.641446]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:35:32 dov kernel: [82080.641449]  [<ffffffff81528739>] ? mutex_lock+0x19/0x50
Dec 21 15:35:32 dov kernel: [82080.641452]  [<ffffffff81528739>] ? mutex_lock+0x19/0x50
Dec 21 15:35:32 dov kernel: [82080.641455]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:35:32 dov kernel: [82080.641470]  [<ffffffffa0085bc6>] i915_gem_throttle_ioctl+0x36/0x90 [i915]
Dec 21 15:35:32 dov kernel: [82080.641483]  [<ffffffffa004bcae>] drm_ioctl+0x17e/0x3a0 [drm]
Dec 21 15:35:32 dov kernel: [82080.641486]  [<ffffffff81011545>] ? __setup_rt_frame+0x2c5/0x420
Dec 21 15:35:32 dov kernel: [82080.641498]  [<ffffffffa00560b0>] ? drm_mode_connector_property_set_ioctl+0x0/0x1c0 [drm]
Dec 21 15:35:32 dov kernel: [82080.641502]  [<ffffffff81036419>] ? default_spin_lock_flags+0x9/0x10
Dec 21 15:35:32 dov kernel: [82080.641504]  [<ffffffff81529b0a>] ? _spin_lock_irqsave+0x2a/0x40
Dec 21 15:35:32 dov kernel: [82080.641507]  [<ffffffff8112e49c>] vfs_ioctl+0x7c/0xa0
Dec 21 15:35:32 dov kernel: [82080.641510]  [<ffffffff8112ea69>] do_vfs_ioctl+0x79/0x370
Dec 21 15:35:32 dov kernel: [82080.641512]  [<ffffffff8112ede1>] sys_ioctl+0x81/0xa0
Dec 21 15:35:32 dov kernel: [82080.641515]  [<ffffffff81012002>] system_call_fastpath+0x16/0x1b
Dec 21 15:35:32 dov kernel: [82080.641520] INFO: task compiz.real:2110 blocked for more than 120 seconds.
Dec 21 15:35:32 dov kernel: [82080.641521] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:35:32 dov kernel: [82080.641523] compiz.real   D 0000000000000000     0  2110   2001 0x00000000
Dec 21 15:35:32 dov kernel: [82080.641526]  ffff88003786bca8 0000000000000082 ffff880000000040 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641529]  ffff8801000c83b0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641532]  0000000000015880 ffff8801000c83b0 0000000000015880 0000000000015880
Dec 21 15:35:32 dov kernel: [82080.641535] Call Trace:
Dec 21 15:35:32 dov kernel: [82080.641538]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:35:32 dov kernel: [82080.641540]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:35:32 dov kernel: [82080.641548]  [<ffffffffa0086c72>] i915_gem_set_domain_ioctl+0xa2/0x140 [i915]
Dec 21 15:35:32 dov kernel: [82080.641558]  [<ffffffffa004bcae>] drm_ioctl+0x17e/0x3a0 [drm]
Dec 21 15:35:32 dov kernel: [82080.641566]  [<ffffffffa0086bd0>] ? i915_gem_set_domain_ioctl+0x0/0x140 [i915]
Dec 21 15:35:32 dov kernel: [82080.641569]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:35:32 dov kernel: [82080.641571]  [<ffffffff8112e49c>] vfs_ioctl+0x7c/0xa0
Dec 21 15:35:32 dov kernel: [82080.641574]  [<ffffffff8112ea69>] do_vfs_ioctl+0x79/0x370
Dec 21 15:35:32 dov kernel: [82080.641577]  [<ffffffff8111f8b1>] ? vfs_read+0x181/0x1a0
Dec 21 15:35:32 dov kernel: [82080.641579]  [<ffffffff8112ede1>] sys_ioctl+0x81/0xa0
Dec 21 15:35:32 dov kernel: [82080.641581]  [<ffffffff81130747>] ? sys_poll+0x77/0x110
Dec 21 15:35:32 dov kernel: [82080.641584]  [<ffffffff81012002>] system_call_fastpath+0x16/0x1b
Dec 21 15:37:32 dov kernel: [82200.641315] INFO: task i915/0:360 blocked for more than 120 seconds.
Dec 21 15:37:32 dov kernel: [82200.641319] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 15:37:32 dov kernel: [82200.641322] i915/0        D 0000000000000000     0   360      2 0x00000000
Dec 21 15:37:32 dov kernel: [82200.641327]  ffff880122dedd70 0000000000000046 0000000000000000 0000000000015880
Dec 21 15:37:32 dov kernel: [82200.641331]  ffff8801234e47c0 0000000000015880 0000000000015880 0000000000015880
Dec 21 15:37:32 dov kernel: [82200.641335]  0000000000015880 ffff8801234e47c0 0000000000015880 0000000000015880
Dec 21 15:37:32 dov kernel: [82200.641339] Call Trace:
Dec 21 15:37:32 dov kernel: [82200.641348]  [<ffffffff81528847>] __mutex_lock_slowpath+0xd7/0x160
Dec 21 15:37:32 dov kernel: [82200.641351]  [<ffffffff81528746>] mutex_lock+0x26/0x50
Dec 21 15:37:32 dov kernel: [82200.641371]  [<ffffffffa00858d8>] i915_gem_retire_work_handler+0x38/0x90 [i915]
Dec 21 15:37:32 dov kernel: [82200.641382]  [<ffffffffa00858a0>] ? i915_gem_retire_work_handler+0x0/0x90 [i915]
Dec 21 15:37:32 dov kernel: [82200.641387]  [<ffffffff810737f5>] run_workqueue+0x95/0x170
Dec 21 15:37:32 dov kernel: [82200.641390]  [<ffffffff81073974>] worker_thread+0xa4/0x120
Dec 21 15:37:32 dov kernel: [82200.641394]  [<ffffffff81078b20>] ? autoremove_wake_function+0x0/0x40
Dec 21 15:37:32 dov kernel: [82200.641398]  [<ffffffff810738d0>] ? worker_thread+0x0/0x120
Dec 21 15:37:32 dov kernel: [82200.641400]  [<ffffffff81078736>] kthread+0xa6/0xb0
Dec 21 15:37:32 dov kernel: [82200.641404]  [<ffffffff810130ea>] child_rip+0xa/0x20
Dec 21 15:37:32 dov kernel: [82200.641408]  [<ffffffff81078690>] ? kthread+0x0/0xb0
Dec 21 15:37:32 dov kernel: [82200.641410]  [<ffffffff810130e0>] ? child_rip+0x0/0x20

My Xorg is very minimal (made by the Ubuntu display preferences, probably):
Section "Device"
	Identifier	"Configured Video Device"
EndSection

Section "Monitor"
	Identifier	"Configured Monitor"
EndSection

Section "Screen"
	Identifier	"Default Screen"
	Monitor		"Configured Monitor"
	Device		"Configured Video Device"
EndSection

Xorg logs have nothing interesting when this happened.