I recently updated to kernel 2.6.31-9.29 and xserver-xorg-video-intel 2:2.8.1-1ubuntu1 (from whatever karmic was about a month ago, which worked fine), and I'm seeing this deadlock as well (on a Thinkpad W500).  I'm also seeing all kinds of display corruption (leftover shadows, images scrambled, etc), but I don't know if it's the same bug or not.

Below is the kernel dmesg output; the X server is locked up and unresponsive, but otherwise the machine is running (accessible via ssh over the network, for instance).  I have an external VGA monitor attached.  So far my machine has locked up twice in a few hours.

[ 5880.448104] INFO: task i915/0:874 blocked for more than 120 seconds.
[ 5880.448113] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 5880.448119] i915/0        D 00000000ffffffff     0   874      2 0x00000000
[ 5880.448131]  ffff880134597d70 0000000000000046 0000000000000000 0000000000015580
[ 5880.448142]  ffff8801348a3110 0000000000015580 0000000000015580 0000000000015580
[ 5880.448151]  0000000000015580 ffff8801348a3110 0000000000015580 0000000000015580
[ 5880.448161] Call Trace:
[ 5880.448180]  [<ffffffff8151f617>] __mutex_lock_slowpath+0xd7/0x160
[ 5880.448190]  [<ffffffff8151f516>] mutex_lock+0x26/0x50
[ 5880.448244]  [<ffffffffa0060768>] i915_gem_retire_work_handler+0x38/0x90 [i915]
[ 5880.448272]  [<ffffffffa0060730>] ? i915_gem_retire_work_handler+0x0/0x90 [i915]
[ 5880.448284]  [<ffffffff8106e805>] run_workqueue+0x95/0x170
[ 5880.448292]  [<ffffffff8106e984>] worker_thread+0xa4/0x120
[ 5880.448300]  [<ffffffff81073b90>] ? autoremove_wake_function+0x0/0x40
[ 5880.448308]  [<ffffffff8106e8e0>] ? worker_thread+0x0/0x120
[ 5880.448315]  [<ffffffff810737a6>] kthread+0xa6/0xb0
[ 5880.448324]  [<ffffffff8101308a>] child_rip+0xa/0x20
[ 5880.448331]  [<ffffffff81073700>] ? kthread+0x0/0xb0
[ 5880.448338]  [<ffffffff81013080>] ? child_rip+0x0/0x20
[ 5880.448365] INFO: task Xorg:3092 blocked for more than 120 seconds.
[ 5880.448369] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 5880.448374] Xorg          D 0000000000000000     0  3092   3085 0x00400004
[ 5880.448384]  ffff88012bc73cb8 0000000000000086 ffff88012bc73c90 0000000000015580
[ 5880.448394]  ffff88012c9d1a60 0000000000015580 0000000000015580 0000000000015580
[ 5880.448403]  0000000000015580 ffff88012c9d1a60 0000000000015580 0000000000015580
[ 5880.448413] Call Trace:
[ 5880.448422]  [<ffffffff8151f617>] __mutex_lock_slowpath+0xd7/0x160
[ 5880.448432]  [<ffffffff814205b0>] ? sock_aio_write+0x0/0x150
[ 5880.448440]  [<ffffffff8151f516>] mutex_lock+0x26/0x50
[ 5880.448467]  [<ffffffffa0060a56>] i915_gem_throttle_ioctl+0x36/0x90 [i915]
[ 5880.448507]  [<ffffffffa0026cae>] drm_ioctl+0x17e/0x3a0 [drm]
[ 5880.448537]  [<ffffffff81032439>] ? default_spin_lock_flags+0x9/0x10
[ 5880.448547]  [<ffffffff815208da>] ? _spin_lock_irqsave+0x2a/0x40
[ 5880.448557]  [<ffffffff81128e2c>] vfs_ioctl+0x7c/0xa0
[ 5880.448566]  [<ffffffff811293f9>] do_vfs_ioctl+0x79/0x370
[ 5880.448574]  [<ffffffff81129771>] sys_ioctl+0x81/0xa0
[ 5880.448585]  [<ffffffff81011fc2>] system_call_fastpath+0x16/0x1b

... and the same message continues every 120 seconds.