Just bought an iMac.
Within a week, was typing along and went for the numbers on the number's keypad - chaos insued.
I'm sure the number's keypad is locked in some type of navigation mode? Maybe?
Anyway, have tried all kinds of combinations with the num-lock button (Not even labled on my keyboard, I'm just assuming it's the one above the "7" marked "Clear."
Have unplugged it. No luck.
Was even weirder when I had this happen on my Apple Wireless Keyboard that has no integrated numeric keypad like the MacBook keyboard on which it's based. You're seeking the Mouse section of the Universal Access pane of System Preferences. Turn off Mouse Keys and, likely, turn off the shortcut to activate it.
- Ben