The man page for smbumount contains this text: "The normal umount program works perfectly well, but it would certainly be problematic to make umount setuid root." umount -is- setuid root. So that non-root users can umount their "user" shares, y'know? I understand that on security-conscious systems where there aren't any "user" shares the setuid bit should be removed, but on every distribution of Linux I've used, the default is to have that bit set..
Fixed - Thanks. I removed this statement. - John T.