The big issue is probably battery life and lightbulb power requirements. Lightbulbs big enough to power projectors heat up a lot, and to have a fan in there too... we're talking some serious weight, in terms of PDAs. But if you could get a good LED projector made up (using LEDs of true red, green, and blue), you might be able to get at least a small screen lit up.

There are now white LEDs that are reasonably powerful. One of these might not prove powerful enough to get a decent sized image but a lens system that could focus several white LED's to a beam that could shine through an imaging system might do the trick.

Send your display data to a glasses type display. See The i-Glasses Store