This is a Prime Sense diagram explaining how their reference platform works. The Kinect is the first (and only) implementation of this platform.
One camera (and one IR transmitter) provide input for the depth map (rumored to be just 320x240), while the third camera detects the human visual spectrum at 640x480 resolution.
We have heard claims that the system can measure distance within 1cm of accuracy at two meters. While that's great in concept, early reviews are showing that you don't get anywhere near that accuracy in the real world—yet.