This video demonstrates making a cheap multitouch control surface, the MTmini:
For this to work, all you need is a room with (at least) some light. The results will be best when the room lighting is even (no bright lights shinning from one direction onto the multitouch pad).
When you place you fingers on the surface, shadows are created where your fingers are. The webcam sees these shadows and sends the image to the tracking software which tracks the shadows.
When you see these things being made out of cardboard boxes, you realize that there’s going to be an explosion of new music interfaces based on multi-touch in the next couple of years.