Same bug here, system was dist-upgraded from intrepid:

[825695.371723] ------------[ cut here ]------------
[825695.371766] kernel BUG at /build/buildd/linux-2.6.28/fs/nfs/write.c:252!
[825695.371795] invalid opcode: 0000 [#1] SMP 
[825695.371823] last sysfs file: /sys/module/ppdev/initstate
[825695.371850] Dumping ftrace buffer:
[825695.371875]    (ftrace buffer empty)
[825695.371898] CPU 4 
[825695.371919] Modules linked in: iptable_filter ip_tables x_tables nfs lockd nfs_acl sunrpc ppdev video output input_polldev aoe 8021q garp stp bonding lp parport psmouse pcspkr iTCO_wdt serio_raw i5100_edac iTCO_vendor_support edac_core joydev xfs usbhid usb_storage e1000e raid10 raid456 async_xor async_memcpy async_tx xor raid1 raid0 multipath linear fbcon tileblit font bitblit softcursor
[825695.372133] Pid: 30470, comm: smbd Not tainted 2.6.28-13-server #45-Ubuntu
[825695.372161] RIP: 0010:[<ffffffffa02b1771>]  [<ffffffffa02b1771>] nfs_page_async_flush+0xa1/0x140 [nfs]
[825695.372225] RSP: 0018:ffff880484e29aa8  EFLAGS: 00010206
[825695.372251] RAX: 0000000000000001 RBX: ffffe200064cdd40 RCX: 0000000000000015
[825695.372294] RDX: 0000000000000000 RSI: 0000000000600020 RDI: ffff8802327a7e5c
[825695.372337] RBP: ffff880484e29ad8 R08: ffff8803e15c1c58 R09: ffff880498c87b80
[825695.372380] R10: 0000000000000000 R11: 0000000000000000 R12: ffff880498c87b80
[825695.372423] R13: ffff8802327a7e5c R14: ffffe200064cdd40 R15: ffff880484e29c48
[825695.372466] FS:  00007f9ac4826710(0000) GS:ffff88061efa2700(0000) knlGS:0000000000000000
[825695.372511] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[825695.372538] CR2: 00007f9abfe481e4 CR3: 000000028c4c1000 CR4: 00000000000006a0
[825695.372581] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[825695.372624] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[825695.372667] Process smbd (pid: 30470, threadinfo ffff880484e28000, task ffff88061cd15980)
[825695.372711] Stack:
[825695.372731]  0007ffffffffffff ffffe200064cdd40 ffff880484e29c48 ffff880484e29b90
[825695.372766]  0007ffffffffffff 0000000000000001 ffff880484e29af8 ffffffffa02b1897
[825695.372816]  ffffe200064cdd40 ffffe200064cdd40 ffff880484e29b18 ffffffffa02b19c9
[825695.372883] Call Trace:
[825695.372905]  [<ffffffffa02b1897>] nfs_do_writepage+0x87/0x90 [nfs]
[825695.372942]  [<ffffffffa02b19c9>] nfs_writepages_callback+0x19/0x30 [nfs]
[825695.372981]  [<ffffffff802b8a28>] write_cache_pages+0x228/0x480
[825695.373013]  [<ffffffffa02b19b0>] ? nfs_writepages_callback+0x0/0x30 [nfs]
[825695.373053]  [<ffffffff80226910>] ? native_flush_tlb_others+0xd0/0xf0
[825695.373085]  [<ffffffff802d26be>] ? free_pages_and_swap_cache+0x7e/0xa0
[825695.373116]  [<ffffffffa02b1053>] nfs_writepages+0xa3/0xf0 [nfs]
[825695.373154]  [<ffffffffa02b1260>] ? nfs_flush_one+0x0/0xf0 [nfs]
[825695.373191]  [<ffffffff802bf31f>] ? vma_prio_tree_next+0x3f/0x60
[825695.373220]  [<ffffffff802b8cd8>] do_writepages+0x28/0x50
[825695.373248]  [<ffffffff802b0c61>] __filemap_fdatawrite_range+0x51/0x60
[825695.373279]  [<ffffffff802b0caa>] filemap_fdatawrite+0x1a/0x20
[825695.373307]  [<ffffffff802b11b5>] filemap_write_and_wait+0x35/0x50
[825695.373336]  [<ffffffffa02a5e23>] nfs_sync_mapping+0x33/0x50 [nfs]
[825695.373373]  [<ffffffffa02a2b57>] do_setlk+0x97/0xe0 [nfs]
[825695.373408]  [<ffffffffa02a2d20>] nfs_lock+0xd0/0x1e0 [nfs]
[825695.373444]  [<ffffffff8031f82e>] vfs_lock_file+0x1e/0x40
[825695.373473]  [<ffffffff8031f958>] do_lock_file_wait+0x48/0xf0
[825695.373501]  [<ffffffff8031fb1c>] fcntl_setlk+0x11c/0x220
[825695.373529]  [<ffffffff802f5b51>] do_fcntl+0x31/0x290
[825695.373557]  [<ffffffff802f5e10>] sys_fcntl+0x60/0x90
[825695.373584]  [<ffffffff8021253a>] system_call_fastpath+0x16/0x1b
[825695.373614] Code: e8 45 de f7 df 90 31 db 48 83 c4 08 89 d8 5b 41 5c 41 5d 41 5e 41 5f c9 c3 90 41 f6 44 24 40 02 74 0d 4c 89 ef e8 20 de f7 df 90 <0f> 0b eb fe 4c 89 f7 e8 43 76 00 e0 85 c0 75 74 49 8b 46 18 ba 
[825695.373806] RIP  [<ffffffffa02b1771>] nfs_page_async_flush+0xa1/0x140 [nfs]
[825695.373845]  RSP <ffff880484e29aa8>
[825695.374178] ---[ end trace 0c6dcbbe446cd8b2 ]---