From page 52 of the new Leopard security guidelines;

Enter a new password and verify it when prompted.
This password can be up to eight characters.
Do not use the capital letter “U” in an Open Firmware password.

According to an Apple support doc (if you do kiss booting your machine goodbye; - darn, not the case)

If you used Open Firmware Password utility to create a password that contains the capital letter "U", your password will not be recognized during the startup process (when you try to access Startup Manager, for example).

Excuse me while I giggle insanely ....