hello,
at my work some people developed this, based on the technology of the electronical instrument "theremin":
the big advantage instead of using optical tracking is that you will have no problems with changing light etc.
another interesting inteface for multitouch is this:
http://www.cycling74.com/products/lemur
but it is very high prised 
when working with Processing have a look to http://webcamxtra.sourceforge.net/
cheers fuxle