This is a very interesting theory, although I just don't see it taking off like a rocket ship. Tech is getting more and more ridiculous each generation and at some point consumers are going to cry fowl and say "Back the F up G, you best recognize!" I can see the appeal in the mid weight and feather weight laptop segments, but I don't ever see it becoming mainstream or even hitting heavy weight laptops. Heavy weights are just that, heavy weights. They have 19" screens, dual GPUs (some), 2 hard drives, and are just plain akward to carry places. Thus, adding on another external device to it seems unreasonable given what they would have already paid for the laptop in the first place.
Now in the middle and light weight segment, this could prove lucrative. Think about it... buy a laptop for school or work that has moderate to decent performance but IGP for battery life, but when you get home you hook up your external GPU and in 5 min you are fraggin the crap outta n00bs. It may not be a huge segment, but it has the potential to combine the power of heavy weight notebooks with the portability and battery life of light weight notebooks. It has to be done right though and it has to work effortlessly. It has to be like plugging in an external hard drive and 1 reboot. There can't be any complicated switches, programs, or other little things that have to be done or it will negate all the convenience.
Another thing to think about is the cooling requirements and the size of the boxes. They will need to be fairly quite (although not silent) and you will need an ac plug in because I don't see this thing being run off of a laptops PSU. Just isn't reasonable. I would suspect that the cards that will really hit home in these break out boxes will be the mid range cards (8800GTS, X1950Pro, etc...), the cards that provide good solid performance but don't put as much heat as the high end gpus (although that is debatable in some product lines).
It is hopeful and a good idea.