It's an interesting question.
I don't believe that this is going to be demand-driven, but more a case of manufacturer-driven market-strategies taht will determine which way things will head.
Assuming that cost of manufacturing sensors will go down over time, one might surmise that every DSLR will eventually become FF and compacts and bridge-cameras may migrate to APS-C/cropped sensors.
On the other hand, is there really a demand for performance that goes much beyond the Nikon D300/Canon 50D level as far as pure sensor-capacity goes? Keeping in mind that the processing chips will improve gradually anyway.
I could see that if the Foveon sensor reach maturity and the cameras that Sigma packs around it also reach more maturity, the big manufacturers might move into similar full-RGB capable sensors as well. In fact, I wouldn't be surprised if Canon and Nikon haven't already begun some serious R&D into this kind of sensor tech. Some of the best showcase-shots from the Foveon has a certain wow-factor that is pretty impressive.
Eventually the consumer-market will be satisfied with regards to mega-pixels. How many enthusiast photograpers require more than say 24MP or 35MP or something like that?
Either way it is going to be an interesting 3-5 years of development coming up on us. In the meantime I'll enjoy my Nikon D40 and it's very delightful performance under most conditions I shoot under anyway..lol.