I was thinking about this with my 18x12 wacom.  All I need is a projector to project onto the tablet and align it so that the cursor matches the image.  In my case the projector is much more expensive than the tablet.  I wonder if it would end up being cheaper than a Cintiq though.  I'd still need to build a frame and such so that the tablet and projector won't move out of alignment.  The above link would also have a bit of problem if someone bumped the tablet.