I saw this a while ago and did a quick test rig to see how well it worked
I just used a pico projector and a laptop to test and use one of them flash webpages that use ur camera to make ripples when movement is seen and it worked well altho the issue was all the flash webpages that detected webcam movement showed the cam image and made ripples of that so seeing your hand on the table then ur projected hand on the table aswell wasn't a good look.
Never did find any programs that used webcam movement but didn't use the webcam image to manipulate so i never bothered to go further with my testing.
I did however play a flash close of advance wars on the tabletop with a led pen and a wiimote which was pretty cool moving units on the table and making them attack