A similar problem has occurred several times with my MacPro. I'm the only user and have never had to login at startup...however, several times since getting my MacPro last year, and for no apparent reason, when I try to do an operation requiring administration password, my user name and password is not accepted.
Using the installation DVD to try to reset the password does not work. the only way to get back on track is to reinstall the operating system, or to copy my backup back to the HD with SuperDuper.
I too am waiting for a solution to this problem which seems to be unique to Leopard on an Intel machine.
Matt