What a cool idea, if done right. I remember 5 or so years ago, IBM made a prototype of something like this - the core of the computer, ie CPU, memory, HD, was a tiny module that could be plugged into different form factors based on the usage you wanted. The capabilities of the two devices seem like they would be so dramatically different though, that I don't see how this would be useful. And if it was useful, how it would be affordable. Maybe the desktop could offload low priority threads to the cpu on the mini-device or something.
The ideal computing set up I would like is a nice desktop (like an iMac) for doing real work and keeping all of my camera docks, printers and other peripherals plugged in, with a light 1kg tablet for portable computing.
The tablet would have a 7-8" touch screen, real keyboard, dual mode display (for ebooks), fantastic calendar software (so it can act as a very efficient day timer), long battery life, syncing as easy as with the iPod. Sell me this whole setup for ~2K, and I'd buy it tomorrow.
Or basically, the offspring of an iPod and an OLPC laptop that go and mate together.
Right now I have a Macbook, which is reasonably OK, but not my idea computing nirvana. It's too heavy, no touch screen, too big to take everywhere, etc.