I can confirm this bug.

# /dev/hda6
UUID=4386-3B67  /media/backup   vfat    defaults,noauto,utf8,umask=007,gid=46,user,noauto 0       1

mount /media/backup

in /proc/mounts
/dev/disk/by-uuid/4386-3B67 /media/backup vfat rw,nosuid,nodev,noexec,uid=1000,gid=46,fmask=0007,dmask=0007,codepage=cp437,iocharset=iso8859-1,utf8 0 0

in /etc/mtab
/dev/hda6 /media/backup vfat rw,noexec,nosuid,nodev,utf8,umask=007,gid=46,user=brian 0 0

umount /media/backup
umount: /media/backup mount disagrees with the fstab

strace umount /media/backup
execve("/bin/umount", ["umount", "/media/backup"], [/* 31 vars */]) = 0
brk(0)                                  = 0x805b000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f91000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=50023, ...}) = 0
mmap2(NULL, 50023, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7f84000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\20Z\1\000"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1248904, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f83000
mmap2(NULL, 1258876, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7e4f000
mmap2(0xb7f7c000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12c) = 0xb7f7c000
mmap2(0xb7f80000, 9596, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb7f80000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7e4e000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb7e4e6b0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xb7f7c000, 8192, PROT_READ)   = 0
munmap(0xb7f84000, 50023)               = 0
brk(0)                                  = 0x805b000
brk(0x807c000)                          = 0x807c000
...
close(3)                                = 0
umask(022)                              = 022
getuid32()                              = 1000
geteuid32()                             = 1000
readlink("/media", 0xbf8ad037, 4096)    = -1 EINVAL (Invalid argument)
readlink("/media/backup", 0xbf8ad037, 4096) = -1 EINVAL (Invalid argument)
umask(077)                              = 022
open("/etc/mtab", O_RDONLY|O_LARGEFILE) = 3
umask(022)                              = 077
fstat64(3, {st_mode=S_IFREG|0644, st_size=623, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7d3f000
read(3, "/dev/hdb1 / ext3 rw,errors=remou"..., 4096) = 623
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0xb7d3f000, 4096)                = 0
stat64("/sbin/umount.vfat", 0xbf8aef6c) = -1 ENOENT (No such file or directory)
oldumount("/media/backup")              = -1 EPERM (Operation not permitted)
oldumount("/dev/hda6")                  = -1 EINVAL (Invalid argument)
lstat64("/etc/mtab", {st_mode=S_IFREG|0644, st_size=623, ...}) = 0
open("/etc/mtab", O_RDWR|O_CREAT|O_LARGEFILE, 0644) = -1 EACCES (Permission denied)
...
write(2, "umount: /dev/hda6: not mounted\n", 31umount: /dev/hda6: not mounted
) = 31
write(2, "umount: /media/backup: must be s"..., 51umount: /media/backup: must be superuser to umount
) = 51
stat64("/sbin/umount.vfat", 0xbf8aef6c) = -1 ENOENT (No such file or directory)
oldumount("/media/backup")              = -1 EPERM (Operation not permitted)
oldumount("/dev/hda6")                  = -1 EINVAL (Invalid argument)
write(2, "umount: /dev/hda6: not mounted\n", 31umount: /dev/hda6: not mounted
) = 31
write(2, "umount: /media/backup: must be s"..., 51umount: /media/backup: must be superuser to umount
) = 51
exit_group(1)                           = ?
Process 7251 detached


sudo umount /media/backup