The photo printer I have doesn't need a computer at all, can just take the memory card out camera stick in the printer and print. The only thing missing to make it totally portable is it still needs household current, figure out a way to run it off battery and walla. And of course there's enough air in the thing a design engineer will eventually figure out a way to merge it with the camera, Digital Polaroid isn't far away.
Battery operated printers exist, but they aren't exactly shaped for carrying around. Something the size and shape of a paperback book would work.