fallsjohn, Disk brakes are pretty common already and I expect moreso in the future because if nothing else the brake system is perhaps percieved as better than rim brakes by many, mostly newer to the sport consumers. No doubt disk brakes can be superior in certain situations such as offroad racing, downhill racing and as a drag brake on a tandem and can be advantageous when riding in the wet. I don't think they'll make rim brakes obsolete in the same manner of the threadless headset's dominance on manufacturing frames.
Brad