This is the third client host I've now had this problem on (the second was a visitor's box, which I no longer have access to).

uname:
Linux llwarchhen 2.6.24-16-generic #1 SMP Thu Apr 10 13:23:42 UTC 2008 i686 GNU/Linux

dmesg:
[ 1106.749396] BUG: unable to handle kernel NULL pointer dereference at virtual address 0000006a
[ 1106.749402] printing eip: c01a31e9 *pde = 00000000 
[ 1106.749407] Oops: 0000 [#1] SMP 
[ 1106.749410] Modules linked in: iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack vmnet(P) vmblock vmmon(P) binfmt_misc af_packet nfsd auth_rpcgss exportfs ppdev autofs4 ipv6 acpi_cpufreq sbs dock cpufreq_powersave cpufreq_conservative cpufreq_userspace container cpufreq_ondemand sbshc cpufreq_stats freq_table nfs lockd nfs_acl sunrpc iptable_filter ip_tables x_tables sbp2 lp pcmcia joydev wacom quickcam videodev v4l2_common v4l1_compat snd_intel8x0 ipw2200 snd_ac97_codec ieee80211 ieee80211_crypt yenta_socket rsrc_nonstatic pcmcia_core ac97_bus nvidia(P) snd_pcm_oss snd_mixer_oss snd_pcm i2c_core snd_seq_dummy video output snd_seq_oss snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq serio_raw iTCO_wdt iTCO_vendor_support irda snd_timer snd_seq_device shpchp button intel_agp battery ac crc_ccitt pci_hotplug agpgart dcdbas parport_pc parport evdev snd soundcore snd_page_alloc psmouse usbhid hid ext3 jbd mbcache sg sr_mod cdrom sd_mod ata_piix ata_generic ohci1394 pata_acpi ieee1394 libata scsi_mod tg3 ehci_hcd uhci_hcd usbcore thermal processor fan fbcon tileblit font bitblit softcursor fuse
[ 1106.749466] 
[ 1106.749469] Pid: 14407, comm: mount.nfs Tainted: P        (2.6.24-16-generic #1)
[ 1106.749471] EIP: 0060:[<c01a31e9>] EFLAGS: 00010206 CPU: 0
[ 1106.749479] EIP is at graft_tree+0x39/0xf0
[ 1106.749481] EAX: f43d1a18 EBX: ffffffec ECX: 00000000 EDX: e0a1df2c
[ 1106.749483] ESI: f6e96e00 EDI: e0a1df2c EBP: 00000023 ESP: e0a1de04
[ 1106.749485]  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
[ 1106.749487] Process mount.nfs (pid: 14407, ti=e0a1c000 task=f5852b40 task.ti=e0a1c000)
[ 1106.749489] Stack: e0a1df2c e0a1df30 f6e96e00 c01a3582 00200006 f6e96e00 e0e90000 e0e29000 
[ 1106.749495]        c01a469b 00000000 e0c05000 00000023 e0a1deac 00000000 e0c6a240 c02e690c 
[ 1106.749499]        00000001 df8cc7e0 df8cc780 f79da6e8 00000025 f42a3180 0000014e f784013c 
[ 1106.749504] Call Trace:
[ 1106.749512]  [<c01a3582>] do_add_mount+0x72/0x100
[ 1106.749522]  [<c01a469b>] do_mount+0x5fb/0x700
[ 1106.749533]  [<c02e690c>] ip4_datagram_connect+0x23c/0x370
[ 1106.749550]  [<c016917f>] find_lock_page+0x2f/0xb0
[ 1106.749561]  [<c016b6c6>] filemap_fault+0x216/0x420
[ 1106.749568]  [<c0216110>] copy_to_user+0x30/0x60
[ 1106.749584]  [<c012103d>] kunmap_atomic+0x3d/0xb0
[ 1106.749601]  [<c01206ad>] fixup_exception+0x1d/0x60
[ 1106.749623]  [<c016e7b0>] __alloc_pages+0x60/0x390
[ 1106.749630]  [<c03190f0>] do_page_fault+0x0/0x730
[ 1106.749654]  [<c01a2fa0>] copy_mount_options+0x40/0x140
[ 1106.749665]  [<c01a4db7>] sys_mount+0x77/0xb0
[ 1106.749676]  [<c01043c2>] sysenter_past_esp+0x6b/0xa9
[ 1106.749691]  [<c0310000>] vcc_getsockopt+0x150/0x170
[ 1106.749706]  =======================
[ 1106.749708] Code: 04 89 c6 89 7c 24 08 8b 40 14 89 d7 8b 40 30 85 c0 79 11 89 d8 8b 74 24 04 8b 1c 24 8b 7c 24 08 83 c4 0c c3 8b 02 b3 ec 8b 48 0c <0f> b7 41 6a 25 00 f0 00 00 3d 00 40 00 00 8b 46 10 0f 94 c2 8b 
[ 1106.749730] EIP: [<c01a31e9>] graft_tree+0x39/0xf0 SS:ESP 0068:e0a1de04
[ 1106.749734] ---[ end trace b56465609cebb5ff ]---