A macbook air running on ARM with a cellular moden is inevitable or at least I hope Apple is thinking the same way I'm thinking :).
I'd be really excited to see an iOS device that integrates in the way that you'e described. Rather than changing the form factor of an iPad, which in my opinion is near-perfect, having it launch OSX or some similar windowed system after recognizing a bluetooth keyboard/mouse, and a monitor connected via the dock connector would be incredible. I would then have a great portable consumption device that I can actually get work done with when I'm in the proper environment.
I just hope Apple doesn't forget about the Unix underneath which is just so helpful to me as a developer.
