Dropped in on an education conference in Canberra today to try a touch screen projector. This is an Epson EB-595Wi short throw projector, which has an addition infra-red detector to turn the display into a touch screen. The clever part of this is that there are no electronics required in the display surface: it can be just a white wall, whiteboard, or whiteboard paint surface (like those of the TEAL room in University of Canberra's Inspire Centre) . You simply point at the image on the wall and the cursor appears there.
The screen size is limited to 100 inches, but as Matthew Brown pointed out when demonstrating the unit, you could not reach the top of a larger screen to operate the touch interface. This unit would be good for smaller classrooms. For very large lecture theatres, the image could be relayed to a large conventional projector (as I set up for the Multinet'95 Conference).
Like other short throw units, this has the limitation you have to bolt the projector to the wall just above where you want the image. But it has the advantage, as with other short throw projectors, that it will work with a shiny whiteboard surface. An additional requirement with the touch unit is that the infra-red detector has to be installed by a technician.