I wonder if there is a technology that allows interaction with a projection. I was looking around the Web but all I could find was fancy and expensive (multi)touch screens.
I think it should be somehow possible to track hand movements from the outside only by means of a little camera and software; similar to the way eye tracking software works.
Imagine you put a little device that is watching how you are doing a presentation by projecting slides on a wall. As soon as you wipe the wall the next slide eases in; you zoom in/out with two hands the same way you achieve that with two fingers on iPhone; when you want to fast forward a movie you wipe with the faster movement; etc.
Probably the challenge would be not to stand in the way of the tracking device and, of course, to clean the wall you've been touching after lounch:)