I'm not sold on the idea of a tablet Mac.
Without any doubt there are some applications where a tablet comes in really handy.
But are you going to input text in these machines? Conventional keyboards are hardly outdone in this regard. I do surprisingly well on my new iPhone but typing on it is still way more tedious and slower than on a dedicated mechanical keyboard (anyone disagree?). A stylus is no viable alternative either... also, Microsoft seems way ahead in terms of Apple in handwriting recognition.
Without an adequate means of inputting text, the whole tablet concept is rather overrated. It sounds / looks better than it actually is, in everyday use. I'd also guess that some of the more sucessful tablets are actually convertibles.
I think, Apple has already placed its bets for the ultraportable market with the iPhone / iPod touch and the MacBook Air. While the iPhone fits into every pocket, the MacBook Air offers portability without compromising on keyboard input and screen real size. An in-between sized "tablet" would compromise on all of these, let alone computing performance!
OK, the MacBook Air doesn't fit the bill (almost literally) for some, because of its somewhat steep price. I think, Apple's will lower the Air's price point considerably. They'll have to! I think it's a little something of future things to come from Apple: Greater portability, more and faster wireless connectivity, less wires, more content and software sold via internet, less need for optical drives.
I would exactly be surprised if the Air or something "Airesque" will have replaced the current MacBook in the consumer market in the next five years or so.