supercaps arepretty awesome. but id guess inductive power transfer. one way to accomplish this is with an inductive mat possibly under or in place of the mouse pad, which powers the mouse through an inductor. mouse would itself be exactly the same as a battery operated mouse, using a bluetooth or other wireless protocal to return mouse data to the computer. however its entirely possible to make a mouse an entirely passive device. whereby it receives a signal though induction, modulates that signal with passive components, and then transmits it back to the pad, much like an rfid tag. there are other methods too, like using a camera to track a mouselike device which is essentially a marker.