It's been done several, dare I say many times. Badly, of course. I saw one at fry's where the base even had a more powerful CPU and when you docked it, the tablet was just the monitor... but you could access the tablet functionality while docked through something that looked like your typical telivisions "Picture in Picture" functionality. There was also a phone, a while back, that would dock into a video/keyboard in a laptop form factor. (Many phones these days, I am given to understand, have hdmi out, so this is largely a mechanical engineering problem. Well, that and making the software usable on both screens.)

So yeah, if it is, in fact, a useful form factor (and I have my doubts) it's in the perfect place in the technology curve for apple; the tech is all there, but nobody has implemented anything usable.