Jeff Han, in the NYU Department of Computer Science has a wonderfully enticing video of a multiple-touch sensor built using the bidirectional capabilities of an array of LEDs. The video is enchanting and worth repeated viewing. The mind boggles at the possibilities. Tragically the minimalist presentation leaves us wondering as to many of the practical aspects of constructing the array. Casual observation shows it is composed of 256 individual sensing elements.
This technical report by Paul Dietz, Darren Leigh and William Yerazunis of Mitsubishi Electric Research Labs offers all the practical knowledge one needs to actually construct something like this. What I wouldn't give for some time to play with such things.