This suggests a problem with the USB port on our computer; try plugging the mouse into the USB port that currently has the keyboard plugged in (obviously, take out the keyboard first) - if mouse becomes unresponsive then you know that it's a USB port problem (on your computer), so you could take it to an Apple store - even if you're out of warranty, or don't use that USB port.
If the mouse works fine whilst in the potentially faulty USB port then plug your keyboard into a different USB port, if the problem continues then you know it's not a problems with the ports on your computer. (To verify).
The next stage is to check the keyboard connections for loose wiring, or fraying of the cable - it is possible that you were unlucky twice.
Then, if possible, dual boot with another OS (xp, Windows vista, Windows 7; there a 30 day activation 'trial' with Windows 7, so you don't need to purchase it for this little test; if not, you could use ubuntu), and (using the same USB port) see if the keyboard is still unresponsive. If it is, then it's a problem with your keyboard; if it's fine then it's a problem with the Mac OS X *driver*.