Hi Stefan, OK.  Here's another stack trace:

[47708.053788] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
[47708.053810] IP: [<ffffffff8130d7f1>] rb_next+0x1/0x50
[47708.053824] PGD 1d0b83067 PUD 1d0e64067 PMD 0 
[47708.053833] Oops: 0000 [#1] SMP 
[47708.053840] CPU 1 
[47708.053843] Modules linked in: ipt_REJECT xt_tcpudp nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack iptable_filter ip_tables x_tables parport_pc ppdev lp parport isofs acpiphp
[47708.053874] 
[47708.053878] Pid: 1190, comm: chef-client Not tainted 3.2.0-24-virtual #37-Ubuntu  
[47708.053888] RIP: e030:[<ffffffff8130d7f1>]  [<ffffffff8130d7f1>] rb_next+0x1/0x50
[47708.053896] RSP: e02b:ffff8801d2e99c18  EFLAGS: 00010046
[47708.053901] RAX: 0000000000000000 RBX: ffff8801d2745800 RCX: 0000000000000000
[47708.053907] RDX: fffffffffffffff0 RSI: 0000000000000000 RDI: 0000000000000010
[47708.053913] RBP: ffff8801d2e99c48 R08: 0000000000000000 R09: 0000000000000000
[47708.053919] R10: ffff8801dff866c0 R11: 0000000000000001 R12: 0000000000000000
[47708.053925] R13: 0000000000000000 R14: 0000000000000000 R15: 000000000240db78
[47708.053935] FS:  00007fc872779700(0000) GS:ffff8801dff8f000(0000) knlGS:0000000000000000
[47708.053943] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[47708.053948] CR2: 0000000000000010 CR3: 00000001d0b3d000 CR4: 0000000000002660
[47708.053955] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[47708.053961] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[47708.053967] Process chef-client (pid: 1190, threadinfo ffff8801d2e98000, task ffff8801d24d2dc0)
[47708.053974] Stack:
[47708.053978]  ffff8801d2e99c48 ffffffff8104ece9 ffff8801d2745800 ffff8801dffa26c0
[47708.053990]  ffff8801d2744a00 0000000000000000 ffff8801d2e99c78 ffffffff810544b8
[47708.054001]  ffff8801d2e99c78 ffff8801dffa26c0 0000000000000001 ffff8801d24d3168
[47708.054013] Call Trace:
[47708.054022]  [<ffffffff8104ece9>] ? pick_next_entity+0xb9/0xe0
[47708.054030]  [<ffffffff810544b8>] pick_next_task_fair+0x38/0x70
[47708.054038]  [<ffffffff81652ddc>] __schedule+0x14c/0x6f0
[47708.054045]  [<ffffffff816554ee>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[47708.054052]  [<ffffffff8165344f>] schedule+0x3f/0x60
[47708.054060]  [<ffffffff8117e119>] pipe_wait+0x59/0x80
[47708.054067]  [<ffffffff81089340>] ? add_wait_queue+0x60/0x60
[47708.054074]  [<ffffffff8117e87a>] pipe_read+0x1da/0x330
[47708.054081]  [<ffffffff81174522>] do_sync_read+0xd2/0x110
[47708.054088]  [<ffffffff8100a25d>] ? xen_force_evtchn_callback+0xd/0x10
[47708.054095]  [<ffffffff8100aa32>] ? check_events+0x12/0x20
[47708.054103]  [<ffffffff81298d33>] ? security_file_permission+0x93/0xb0
[47708.054110]  [<ffffffff811749a1>] ? rw_verify_area+0x61/0xf0
[47708.054116]  [<ffffffff81174e80>] vfs_read+0xb0/0x180
[47708.054122]  [<ffffffff81174f9a>] sys_read+0x4a/0x90
[47708.054129]  [<ffffffff8165d8c2>] system_call_fastpath+0x16/0x1b
[47708.054134] Code: 89 06 48 8b 47 08 48 89 46 08 48 8b 47 10 48 89 46 10 c3 0f 1f 80 00 00 00 00 48 89 32 eb b2 0f 1f 00 48 89 70 10 eb a9 66 90 55 <48> 8b 17 48 89 e5 48 89 d0 48 83 e0 fc 48 39 c7 74 34 48 8b 47 
[47708.054224] RIP  [<ffffffff8130d7f1>] rb_next+0x1/0x50
[47708.054231]  RSP <ffff8801d2e99c18>
[47708.054235] CR2: 0000000000000010
[47708.054253] ---[ end trace 3fc2767cd5259b41 ]---

This is, of course still on EC2.  Here's the Xen version info from the system log:

Xen version: 3.4.3-2.6.18 (preserve-AD)

Hope that helps!