We had a strange thing happen to the two machines that we upgraded to Snow Leopard. Both machines had this happen about a week apart.
Upon boot up, the log in screen appears (both machines have one user, so this never occurs) prompting for a password. When entering the password, the Mac would not accept it and kept asking for the password.
We had to do a hard reboot (pressing and holding the on/off button) until the machine shut down. Then on reboot everything was ok. Kind of scary at first, but it has not happened again.