Help! My "5" key doesn't work, but "%" does??

I've got an interesting problem. Seems that the "5" key at the top of my Aluminum keyboard doesn't work, but using that same key with modifiers such as shift to type the "%" sign does work. The number keypad on the right side of the keyboard works fine for all numbers including 5.

Could I have accidentally mapped the "5" key to a keyboard shortcut or something? I've poked around the System Preferences to try and track this down, with no luck. Any suggestions would be appreciated, as this is driving me nuts!

10.5.2
24" iMac
Aluminum Keyboard

Thanks,