There's actually a quite nifty principle that makes it relatively easy to build your own multitouch table: Frustrated total internal reflection.
The idea is based on internal reflection: Whenever lights hits the boundary between two materials with different density it gets refracted. If it goes from dense to less dense and the angle is flat enough it will reflect back. This is the principle behind fibre optics. It's also the reason why the water's surface looks "silvery" (like a mirror) when you're submerged and look up at an angle (i.e. not straight up).
Now imagine you have a plane of glass and you put LEDs around the edges that shine into it from the side. The light will zig-zag trough the glass and come out at the other edge. However, if you touch the glass you inhibit that total internal reflection because your finger is alot denser than the air and so the light will "leak" out the glass where you touch it, illuminating your finger. If you look at the glass from behind you'll see a bright spot.
Use a camera to detect that spot and you basically have a touchpad. To make it a screen you can put a translucent sheet behind the glass and project an image onto it (and use infrared LEDs).
See e.g. http://wiki.nuigroup.com/FTIR for some helpful images. Just google "FTIR touch screen" or similar for build instructions and blob-detection software.