The Magic Trackpad is basically a larger version of the MacBook Pro touchpad, with 80% more surface area for all your swiping and pinching. The entire surface acts as a button, so it's also a possible mouse replacement. And all of the expected gestures are here: two-finger scrolling, pinch to zoom, fingertip rotation, and three- and four-finger swipes.
It also works up to 33 feet away from your computer, apparently.
Which raises the question: Why would you ever want to work so far away from your computer?
Would you even be able to see the screen? Probably not.
Anyway. It also, like the wireless mouse and keyboard, runs on batteries, and Apple also announced a battery charger.