It happens to me with latest kernel 2.6.20-16-generic from feisty :

$ uname -a
Linux nanook 2.6.20-16-generic #2 SMP Wed May 23 01:46:23 UTC 2007 i686 GNU/Linux

Just after using my NFSv4 share, I got this:

[83308.164030] BUG: unable to handle kernel NULL pointer dereference at virtual address 0000000c
[83308.164041]  printing eip:
[83308.164044] f98878ef
[83308.164046] *pde = 00000000
[83308.164051] Oops: 0000 [#1]
[83308.164053] SMP 
[83308.164058] Modules linked in: nls_cp437 isofs udf binfmt_misc rfcomm l2cap bluetooth nfs lockd sunrpc autofs4 radeon drm speedstep_lib cpufreq_userspace cpufreq_stats cpufreq_powersave cpufreq_ondemand freq_table cpufreq_conservative tc1100_wmi pcc_acpi dev_acpi sony_acpi video sbs i2c_ec dock button battery container ac asus_acpi backlight af_packet sr_mod sbp2 parport_pc lp parport snd_atiixp snd_ac97_codec ac97_bus snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy snd_seq_oss snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq sg snd_timer snd_seq_device sd_mod snd i2c_piix4 soundcore i2c_core usblp psmouse snd_page_alloc serio_raw pcspkr shpchp pci_hotplug ati_agp agpgart evdev tsdev ext3 jbd mbcache ide_cd cdrom ide_disk ata_generic libata 8139too generic usb_storage scsi_mod libusual usbhid hid raid10 ohci1394 ieee1394 ehci_hcd atiixp 8139cp mii ohci_hcd usbcore raid456 xor raid1 raid0 multipath linear md_mod thermal processor fan dm_mod fbcon tileblit font bitblit softcursor vesafb capability commoncap
[83308.164211] CPU:    1
[83308.164212] EIP:    0060:[<f98878ef>]    Not tainted VLI
[83308.164214] EFLAGS: 00010246   (2.6.20-16-generic #2)
[83308.164239] EIP is at nfs_update_inode+0xbf/0x6a0 [nfs]
[83308.164243] eax: 00000000   ebx: 000081a4   ecx: 000081a4   edx: 00008000
[83308.164248] esi: 0191491e   edi: f3ef73c0   ebp: d75d31dc   esp: c8ba1de0
[83308.164252] ds: 007b   es: 007b   ss: 0068
[83308.164256] Process umount (pid: 17395, ti=c8ba0000 task=caa8f030 task.ti=c8ba0000)
[83308.164259] Stack: c02ed98b 00000001 f54f62e8 00000004 00000000 caa8f030 c013ae50 cfee00b0 
[83308.164271]        d75d30b4 c8ba1e20 d75d31dc c0130ec5 f54f6280 00000000 d75d31dc cfee00b0 
[83308.164284]        f54f6280 d75d31dc f9889224 cfee0000 00000000 f98979c2 cfee0000 cf58a510 
[83308.164296] Call Trace:
[83308.164299]  [<c02ed98b>] out_of_line_wait_on_bit+0x7b/0x90
[83308.164326]  [<c013ae50>] wake_bit_function+0x0/0x60
[83308.164346]  [<c0130ec5>] sigprocmask+0x65/0x100
[83308.164371]  [<f9889224>] nfs_post_op_update_inode+0x24/0x50 [nfs]
[83308.164397]  [<f98979c2>] nfs4_proc_delegreturn+0x192/0x1a0 [nfs]
[83308.164458]  [<f98a34d7>] nfs_do_return_delegation+0x17/0x30 [nfs]
[83308.164491]  [<f988615c>] nfs_dentry_iput+0x1c/0x50 [nfs]
[83308.164515]  [<c0187a52>] shrink_dcache_for_umount_subtree+0x82/0x230
[83308.164538]  [<c011e0d9>] __wake_up_common+0x39/0x60
[83308.164562]  [<c0188817>] shrink_dcache_for_umount+0x37/0x50
[83308.164571]  [<c0178318>] generic_shutdown_super+0x18/0xf0
[83308.164589]  [<c0178439>] kill_anon_super+0x9/0x40
[83308.164599]  [<f9889ddc>] nfs_kill_super+0xc/0x20 [nfs]
[83308.164621]  [<c01784ed>] deactivate_super+0x5d/0x80
[83308.164633]  [<c018cdd2>] expire_mount_list+0xc2/0x140
[83308.164664]  [<c018de73>] shrink_submounts+0xb3/0xd0
[83308.164697]  [<c018db69>] sys_umount+0x109/0x270
[83308.164748]  [<c02f079f>] do_page_fault+0x33f/0x5f0
[83308.164792]  [<c018dce7>] sys_oldumount+0x17/0x20
[83308.164804]  [<c01031f0>] sysenter_past_esp+0x69/0xa9
[83308.164865]  =======================
[83308.164868] Code: 00 00 00 02 89 e8 e8 d1 fc ff ff b8 8c ff ff ff 83 c4 38 5b 5e 5f 5d c3 8d 74 26 00 8b 85 98 00 00 00 8b b8 60 01 00 00 8b 40 38 <3b> 68 0c 0f 84 2b 04 00 00 8b 54 24 1c 8b 4c 24 20 8b 82 8c 00 
[83308.164934] EIP: [<f98878ef>] nfs_update_inode+0xbf/0x6a0 [nfs] SS:ESP 0068:c8ba1de0
[83308.164954]  <1>BUG: unable to handle kernel NULL pointer dereference at virtual address 0000000c
[83368.076185]  printing eip:
[83368.076188] f98878ef
[83368.076190] *pde = 00000000
[83368.076194] Oops: 0000 [#2]
[83368.076196] SMP 
[83368.076200] Modules linked in: nls_cp437 isofs udf binfmt_misc rfcomm l2cap bluetooth nfs lockd sunrpc autofs4 radeon drm speedstep_lib cpufreq_userspace cpufreq_stats cpufreq_powersave cpufreq_ondemand freq_table cpufreq_conservative tc1100_wmi pcc_acpi dev_acpi sony_acpi video sbs i2c_ec dock button battery container ac asus_acpi backlight af_packet sr_mod sbp2 parport_pc lp parport snd_atiixp snd_ac97_codec ac97_bus snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy snd_seq_oss snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq sg snd_timer snd_seq_device sd_mod snd i2c_piix4 soundcore i2c_core usblp psmouse snd_page_alloc serio_raw pcspkr shpchp pci_hotplug ati_agp agpgart evdev tsdev ext3 jbd mbcache ide_cd cdrom ide_disk ata_generic libata 8139too generic usb_storage scsi_mod libusual usbhid hid raid10 ohci1394 ieee1394 ehci_hcd atiixp 8139cp mii ohci_hcd usbcore raid456 xor raid1 raid0 multipath linear md_mod thermal processor fan dm_mod fbcon tileblit font bitblit softcursor vesafb capability commoncap
[83368.076317] CPU:    1
[83368.076318] EIP:    0060:[<f98878ef>]    Not tainted VLI
[83368.076319] EFLAGS: 00010246   (2.6.20-16-generic #2)
[83368.076339] EIP is at nfs_update_inode+0xbf/0x6a0 [nfs]
[83368.076342] eax: 00000000   ebx: 000081a4   ecx: 000081a4   edx: 00008000
[83368.076346] esi: 019148f4   edi: f3ef73c0   ebp: f0d31644   esp: f0387f1c
[83368.076349] ds: 007b   es: 007b   ss: 0068
[83368.076352] Process 192.168.0.2-del (pid: 17467, ti=f0386000 task=cda61030 task.ti=f0386000)
[83368.076354] Stack: c02ed98b 00000001 f54f64e8 00000004 00000000 cda61030 c013ae50 dbbc04b0 
[83368.076364]        f0d3151c f0387f5c f0d31644 c0130ec5 f54f6480 00000000 f0d31644 dbbc04b0 
[83368.076373]        f54f6480 f0d31644 f9889224 dbbc0400 00000000 f98979c2 dbbc0400 ee9bcf10 
[83368.076382] Call Trace:
[83368.076385]  [<c02ed98b>] out_of_line_wait_on_bit+0x7b/0x90
[83368.076407]  [<c013ae50>] wake_bit_function+0x0/0x60
[83368.076424]  [<c0130ec5>] sigprocmask+0x65/0x100
[83368.076446]  [<f9889224>] nfs_post_op_update_inode+0x24/0x50 [nfs]
[83368.076466]  [<f98979c2>] nfs4_proc_delegreturn+0x192/0x1a0 [nfs]
[83368.076515]  [<f98a34d7>] nfs_do_return_delegation+0x17/0x30 [nfs]
[83368.076539]  [<f98a3d77>] nfs_do_expire_all_delegations+0x77/0xa0 [nfs]
[83368.076568]  [<f98a3d00>] nfs_do_expire_all_delegations+0x0/0xa0 [nfs]
[83368.076585]  [<c013ac4a>] kthread+0xba/0xf0
[83368.076594]  [<c013ab90>] kthread+0x0/0xf0
[83368.076606]  [<c01044c7>] kernel_thread_helper+0x7/0x10
[83368.076628]  =======================
[83368.076630] Code: 00 00 00 02 89 e8 e8 d1 fc ff ff b8 8c ff ff ff 83 c4 38 5b 5e 5f 5d c3 8d 74 26 00 8b 85 98 00 00 00 8b b8 60 01 00 00 8b 40 38 <3b> 68 0c 0f 84 2b 04 00 00 8b 54 24 1c 8b 4c 24 20 8b 82 8c 00 
[83368.076681] EIP: [<f98878ef>] nfs_update_inode+0xbf/0x6a0 [nfs] SS:ESP 0068:f0387f1c
[83368.076696]  

Attached is my dmesg.log, for information.