A do-it-yourselfer noticed that the LED on the new Mighty Mouse "would blink when it is not in sync with a computer. Once sync'ed, the LED would go OFF if the mouse is moving and it would be continually ON when the mouse is NOT moving." He then made it his duty to take out the LED from the mouse, seeing how much power that would save.
Funny thing is that after syncing, the mouse seems to using less power when the mouse is moving (!), i.e. when the LED is off (~.0285 Amps) versus when it is not moving and the LED is on (~.0450 Amps).
Quite a lot, it seems. After the procedure was finished, people pointed out that the light only came on because the laser was out-of-focus, and that his procedure didn't save quite as much power on the mouse as he had thought. It still extends the life of the battery, just not as dramatically as the 0.0450 vs 0.0285 figures.
Check out the link to see how to do this to your mouse.