I don't have any of their products, but I've heard of the wireless charging mat for the iPhone. While this looks interesting, I use two pair of NiMH AA cells in rotation for my Magic Trackpad. I get maybe two - three months between changing the AA cells and putting them in the charger for a day.
What I wouldn't mind wound be a USB-wired replacement for the battery compartment on the Magic Trackpad to eliminate the swapping. No need to carry signaling over the USB, just use it for powering the trackpad and the normal Bluetooth would do the rest.