Using the Kinect for Windows Beta from Nov 2011 and a Kinect sensor, you register this guy's driver with Windows and then Skype can see the Kinect as a webcam!
Infrared light from a laser(s) is shined just above the surface. The laser plane of light is about 1mm thick and is positioned right above the surface, when the finger just touches it, it will hit the tip of the finger which will register as a IR blob.