This video shows a demonstration of Bonfire, an additional interface for computers. It consists of a pico projector and camera hang on the back of either side of a laptop. The projector displays information on the table top and the camera monitors the area for interaction. It can recognize your hand or objects such as a smartphone or headphones and react accordingly. An accelerometer in the laptop picks up tapping (we’d guess you have to tap pretty hard) and there is also support for gestures. This was presented at 2009 UIST and unfortunately the published article is not available for free [Thanks for the link Ciflet].
We hope to see this kind of thing, as well as skin based input, come to the market some day. Until then, you’ll have to build your own.