I'm not going to say it won't happen, but I'd be really surprised if Apple jumped into this market.
The Tablet PC, by and large, has been a commercial failure. Microsoft has stirred up the concept several times in the last 15 years, and each incarnation has pretty much shown that the ergonomics, UI and practicality are highly questionable. With multitouch, gestures, an accelerometer and a much better OS to start with, I can see where Apple might have an advantage. But I really think the concept is fundamentally flawed.
You have your iPhone-size device, which is almost as powerful as a notebook. You can do most of your core functionality from there (mail, contacts, appointments, browsing, mail, some games, etc). Then you have a notebook which can do just about anything else besides hard core tasks (like heavy image processing, video editing, etc). Between those two devices, I'm just not seeing a niche that needs to be filled. Perhaps I'm missing something.
I don't know if any of you that are excited about this have used a Tablet PC before, but I have - extensively. While the stylus and handwriting recognition are novel, it quickly wears off and you find yourself wanting to use it like a laptop. Especially since you can get a laptop in the same formfactor that has a standard, albeit smaller, keyboard and trackpad.