Allan,
Actually funny you should say "any Mac USB keyboard." Apple made some newer keyboards that may only function after booting via open firmware
if something is wrong with the PRAM battery. Some of the flat key keyboards are that way. I had that experience with an iMac G5. Once I typed mac-boot it worked for the most part, though sometimes I had to also force quit the Finder to regain access to all the other functions of the keyboard. I don't have the time get a new PRAM battery. So that's just something to keep in mind. MacAlly and Logitech also make Mac keyboards and may have drivers that work better with PowerPC Mac operating systems.