This forum has included a few posts linking to "projection keyboards", i.e. little camera thingies that shine something I call "lay-zers" onto a tabletop, and that scan your fingers for "keypresses".
Well, even cooler: check out Microsoft's "PlayAnywhere" prototype demo... "A Compact Interactive Table Top Projection-Vision System" (and it only uses a single camera, yet can determine "depth"!)
http://www.videosift.com/?domains=www.videosift.com&search=PlayAnywhere+Interactive+Projector+&sa=Search
(some other cool videos show up in the above search too 8) )