[ww520]

Interesting problem. Haven't thought of the sampling rate required. The location resolution probably will be poor for precise multitouch function. But even a single 2D location is enormously useful - turning any surface into a touch screen.

kabdib mentioned phrased array, which can be very helpful. It might solve the sampling rate issue since it's analog. I found this http://en.wikipedia.org/wiki/Beamforming. May be some of the techniques in radar/sonar can be borrowed here.