Great idea, I did something in the past with having two MOBOs in one case just to save space, one running a dedicated media server and the other my normal computer. So something like this definitely has a market.
It is a nice idea, makes me want one to toggle separate pc's in my house. As it stands however, I don't see this being /that/ useful at the rate that computers are these days. Still sounds like something fun to mess around with none the less.