We have had a crash with Stefan's debugging kernel running.  Here is the output (doesn't look like it contains any more information).

[248587.286290] ------------[ cut here ]------------
[248587.286765] kernel BUG at /home/smb/precise-amd64/ubuntu-2.6/kernel/sched_fair.c:1239!
[248587.286775] invalid opcode: 0000 [#1] SMP 
[248587.286783] CPU 0 
[248587.286786] Modules linked in: ipt_REJECT xt_tcpudp nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack iptable_filter ip_tables x_tables bnep rfcomm bluetooth parport_pc ppdev lp parport isofs acpiphp
[248587.286822] 
[248587.286827] Pid: 18805, comm: ohai Not tainted 3.2.0-24-virtual #37+lp999755dbg1  
[248587.286836] RIP: e030:[<ffffffff81050cd5>]  [<ffffffff81050cd5>] pick_next_entity+0x105/0x110
[248587.286849] RSP: e02b:ffff8801d02d7c28  EFLAGS: 00010096
[248587.286854] RAX: 000000000000002d RBX: ffff8801d20df800 RCX: 000000000003ffff
[248587.286860] RDX: 0000000000000000 RSI: ffffffff81e000a0 RDI: 0000000000000004
[248587.286866] RBP: ffff8801d02d7c48 R08: 000000000000000a R09: 0000000000000000
[248587.286872] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801dff866c0
[248587.286878] R13: ffff8801d20de600 R14: 0000000000000000 R15: 0000000001f41018
[248587.286889] FS:  00007f5ec2010700(0000) GS:ffff8801dff73000(0000) knlGS:0000000000000000
[248587.286895] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[248587.286901] CR2: 0000000001f09f30 CR3: 00000001cec3e000 CR4: 0000000000002660
[248587.286908] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[248587.286914] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[248587.286921] Process ohai (pid: 18805, threadinfo ffff8801d02d6000, task ffff8801cf10db80)
[248587.286928] Stack:
[248587.286931]  ffff8801d20df800 ffff8801dff866c0 ffff8801d20de600 0000000000000000
[248587.286944]  ffff8801d02d7c78 ffffffff810544e8 ffff8801d02d7c78 ffff8801dff866c0
[248587.286956]  0000000000000000 ffff8801cf10df28 ffff8801d02d7cf8 ffffffff81652f3c
[248587.286968] Call Trace:
[248587.286976]  [<ffffffff810544e8>] pick_next_task_fair+0x38/0x70
[248587.286984]  [<ffffffff81652f3c>] __schedule+0x14c/0x6f0
[248587.286992]  [<ffffffff8165564e>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[248587.286999]  [<ffffffff816535af>] schedule+0x3f/0x60
[248587.287006]  [<ffffffff8117e149>] pipe_wait+0x59/0x80
[248587.287014]  [<ffffffff81089370>] ? add_wait_queue+0x60/0x60
[248587.287021]  [<ffffffff8117e8aa>] pipe_read+0x1da/0x330
[248587.287028]  [<ffffffff81174552>] do_sync_read+0xd2/0x110
[248587.287036]  [<ffffffff8100a25d>] ? xen_force_evtchn_callback+0xd/0x10
[248587.287043]  [<ffffffff8100aa32>] ? check_events+0x12/0x20
[248587.287051]  [<ffffffff81298d63>] ? security_file_permission+0x93/0xb0
[248587.287057]  [<ffffffff811749d1>] ? rw_verify_area+0x61/0xf0
[248587.287063]  [<ffffffff81174eb0>] vfs_read+0xb0/0x180
[248587.287069]  [<ffffffff81174fca>] sys_read+0x4a/0x90
[248587.287076]  [<ffffffff8165da42>] system_call_fastpath+0x16/0x1b
[248587.287081] Code: 89 df e8 69 c1 5e 00 48 8b 73 38 48 c7 c7 7b ef 9f 81 31 c0 e8 98 c9 5e 00 48 8b 73 10 48 c7 c7 98 ef 9f 81 31 c0 e8 86 c9 5e 00 <0f> 0b 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 48 83 ec 10 48 89 
[248587.287170] RIP  [<ffffffff81050cd5>] pick_next_entity+0x105/0x110
[248587.287177]  RSP <ffff8801d02d7c28>
[248587.287194] ---[ end trace 124f7d4d99f55a46 ]---