that's pretty cool...
it's not going to work for your idea though, as you pointed out, if there is a projector in the box then the light source will not allow the shaddows to pass though the box. (which is what the webcam is detecting to allow the software to work).
what you will need is a real touch screen on top of the table.
something like this
22 Inch 4 Wire Resistive Touch Screen Panel Kit for PC on eBay (end time 27-Feb-11 14:31:47 GMT)
alternativly, with some mirrors and focusing lenses you may be able to take your projector image and display it from the top.
(but then your fingers are going to cast shaddows and obstruct the picture unless you can cast the image from several light sources (projectors or mirrors) at once).
in short... your options for making a touch screen coffee table are.
buy a touch screen monitor and encase it in wood.
or make a projection box and put a touch screen matrix on top of it.
the first option will be the most reliable,
the second option will be the most work.