BT Mightymouse please ..
Seriously.. I've had this idea for over a year now.
Since the first BT mouse was released and i went through a 6 pack of AAs in a week. -_-
It's quite simple. Your mouse gets low? You plug the firewire cable into the front of the mouse, and into the computer. You then continue using it as you would a wired mouse, as it charges.
It doesn't take a frickin genius.. come on Apple... -_-
[Edit: Fixed image link (Anders)]