For me this actually manages to trigger an oops after a zillion

VFS: busy inodes on changed media or resized disk sr1

This is on 2.6.38-10-generic #44-Ubuntu SMP and also on a different machine (with the same USB stick) on a 3.0rc3 kernel I built with a similar backtrace.

[17175.349990] BUG: unable to handle kernel NULL pointer dereference at 0000000000000002
[17175.350050] IP: [<ffffffff81a255c2>] init_groups+0x2/0xa0
[17175.350092] PGD 17e1d1067 PUD 1ea6a8067 PMD 0 
[17175.350130] Oops: 0002 [#1] SMP 
[17175.350157] last sysfs file: /sys/devices/pci0000:00/0000:00:1a.0/usb1/1-1/1-1.4/1-1.4:1.0/bInterfaceSubClass
[17175.350214] CPU 6 
[17175.350227] Modules linked in: nls_utf8 isofs sha256_generic cryptd aes_x86_64 aes_generic ip6table_filter ip6_tables binfmt_misc dm_crypt ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack ipt_REJECT xt_CHECKSUM iptable_mangle xt_tcpudp iptable_filter ip_tables x_tables bridge stp kvm_intel kvm snd_hda_codec_hdmi snd_hda_codec_via snd_hda_intel snd_hda_codec snd_hwdep snd_seq_midi snd_pcm snd_rawmidi ppdev snd_seq_midi_event snd_seq snd_timer snd_seq_device lp snd soundcore snd_page_alloc parport_pc parport radeon ttm usbhid firewire_ohci usb_storage uas hid r8169 firewire_core drm_kms_helper pata_via crc_itu_t drm i2c_algo_bit configfs
[17175.350740] 
[17175.350752] Pid: 1779, comm: umount Tainted: G        W   2.6.38-10-generic #44-Ubuntu To Be Filled By O.E.M. To Be Filled By O.E.M./P55M Pro
[17175.350838] RIP: 0010:[<ffffffff81a255c2>]  [<ffffffff81a255c2>] init_groups+0x2/0xa0
[17175.350891] RSP: 0018:ffff88014d0b3b30  EFLAGS: 00010002
[17175.350924] RAX: 0000000000000002 RBX: ffff88017e0686d0 RCX: 0000000000000010
[17175.350968] RDX: ffffffff81a255c0 RSI: 0000000000000000 RDI: ffff88017e0686d0
[17175.351011] RBP: ffff88014d0b3b38 R08: 0000000000000000 R09: ffff88021a5878a0
[17175.351053] R10: ffff88021a5878a0 R11: 0000000000000005 R12: 0000000000000000
[17175.351096] R13: 0000000000000001 R14: ffff88014d0b3d01 R15: ffff88021a5878a0
[17175.351141] FS:  00007eff431d5760(0000) GS:ffff8800c7580000(0000) knlGS:0000000000000000
[17175.351191] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[17175.351224] CR2: 0000000000000002 CR3: 000000014d94e000 CR4: 00000000000006e0
[17175.351267] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[17175.351311] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[17175.351355] Process umount (pid: 1779, threadinfo ffff88014d0b2000, task ffff8801436b8000)
[17175.351405] Stack:
[17175.351418]  ffffffff812bd1ed ffff88014d0b3b98 ffffffff812c2e18 ffff88014d0b3b78
[17175.351471]  0000000000000082 0000000000000000 ffff880100000010 0000000000000000
[17175.351520]  ffff88017e0686d0 0000000000000000 0000000000000010 ffff88014d0b3db8
[17175.351573] Call Trace:
[17175.351594]  [<ffffffff812bd1ed>] ? elv_may_queue+0x1d/0x20
[17175.351628]  [<ffffffff812c2e18>] get_request+0x48/0x3f0
[17175.351664]  [<ffffffff812c3479>] get_request_wait+0x29/0x1a0
[17175.351702]  [<ffffffff8110c6b0>] ? find_get_pages_tag+0x40/0x120
[17175.351744]  [<ffffffff8105f464>] ? try_to_wake_up+0x244/0x3e0
[17175.351778]  [<ffffffff812c365c>] blk_get_request+0x6c/0xa0
[17175.351811]  [<ffffffff813e5888>] scsi_execute+0x48/0x160
[17175.351842]  [<ffffffff813e652a>] scsi_execute_req+0xca/0x140
[17175.351877]  [<ffffffff813dfdb1>] ioctl_internal_command.clone.4+0x61/0x1b0
[17175.351917]  [<ffffffff8119f045>] ? fsnotify_clear_marks_by_inode+0x95/0xf0
[17175.351957]  [<ffffffff813dff7e>] scsi_set_medium_removal+0x7e/0xb0
[17175.351993]  [<ffffffffa040171c>] ? isofs_destroy_inode+0x1c/0x20 [isofs]
[17175.352032]  [<ffffffff813f7020>] sr_lock_door+0x20/0x30
[17175.352064]  [<ffffffff8142d3d7>] cdrom_release+0xe7/0x140
[17175.352094]  [<ffffffff813f5b88>] sr_block_release+0x38/0x60
[17175.352124]  [<ffffffff811994bc>] __blkdev_put+0x16c/0x1b0
[17175.352154]  [<ffffffff811995d2>] blkdev_put+0xd2/0x170
[17175.352183]  [<ffffffff81166d3e>] kill_block_super+0x4e/0x80
[17175.352213]  [<ffffffff811671f5>] deactivate_locked_super+0x45/0x70
[17175.352245]  [<ffffffff81167e7a>] deactivate_super+0x4a/0x70
[17175.352276]  [<ffffffff81183294>] mntput_no_expire+0xa4/0xf0
[17175.352306]  [<ffffffff81184320>] sys_umount+0x60/0xd0
[17175.352333]  [<ffffffff8100c002>] system_call_fastpath+0x16/0x1b
[17175.352364] Code: 00 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 a8 55 a2 81 ff ff ff ff a8 55 a2 81 ff ff ff ff 00 00 00 00 00 00 00 00 34 02 <00> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
[17175.352631] RIP  [<ffffffff81a255c2>] init_groups+0x2/0xa0
[17175.352663]  RSP <ffff88014d0b3b30>
[17175.352681] CR2: 0000000000000002
[17175.365957] ---[ end trace 7664560b4e26c23d ]---
[17195.912249] usb 1-1.4: USB disconnect, address 3
root@major:~#