I have just finished my own multitouch setup for the community college that I work for. It uses front illumination as I have had the most reliably results with it. It has a Phillips SPC900NC webcam with the special lens and film negatives to block visible light. There is a projector and mirror inside the box and a piece of vellum on the screen to catch the image and diffuse infrared light. I gave a long demo of it and used touchlib to send data via osc and had WorldWind track blobs and use it to view the world.

http://www.geckoprojects.com/screen/