They need a receiver on that surface, usually hooked up by bluetooth or usb. Set up heaps of them.
Edit: are you talking about those dell units? Not played with one of them. If they are like their earlier models, putting interactive on a dell projector is like putting sauce on a shit sandwich.
Much nicer unit. The interactive receiver would be built into the projector itself. High lumens so would be very forgiving in a bright room. I still expect it would suck though with motion over wireless.
Video over wireless is fine, audio over wireless through projector is ever so slightly out of sync. But you can connect to your own speakers and fix this.
You can annotate (draw) with nothing connected to the projector or while connected over wireless but won't be able to save the annotation, if projector is plugged via usb in you can save the annotation, use the pens as a mouse and use 2 pens at once :). Also has limited iPad support.
Overall I think its pretty good, much better and around half the cost of interactive boards. Going to get half a dozen more now.
All they have to do is make all the above work wirelessly and these would be perfect. Would be nice if they bundled the 3 installers into 1.