While a device of this type does appeal to a smaller market (i.e., artists, photographers, etc), it's that market that Apple has been strong with for years, even before it's resurgence. As an artist myself, I was told for years that if you are going to do anything graphic-wise with a computer, Apple was the way to go.
The problem is that the market is so small the prospect of making a profit is tiny, I'm a photographer and I have 0 interest in using a tablet pc (regardless of who makes it). While there will be people who want or clamor for one, they're such a minority that I believe that apple will hard pressed to recoup the R&D, marketing and manufacturing cost. Apple does appeal to some niche markets but in those markets that it beleives it can make money. That doesn't mean it should enter every niche market. I think the tablet computer is one such market that it would best avoid imho.
The PC tablets have been pretty lackluster in terms of acceptance, but if you look at the specs of those tablets, you can see why creative professionals probably stayed away. I looked at them long and hard and decided that it was better to get an external Wacom tablet that had all the features I wanted, rather than go with an expensive tablet that only half-delivered the same abilities. PC tablets were built around the idea of a "power user" being able to take it into a meeting room or wherever and use it for business means. In typical Microsoft fashion, they misjudged the market this device would appeal to.
I don't believe the power of the computer was its downfall but rather the ease of use. Its still much easier and less error prone to enter data via a keyboard.
It is easy to think of a tablet computer as a device that would only appeal to a small niche market. However, take into account that the market in question could include 3d artists working for special effects studios, concept artists for games or movies, as well as independent artists around the world. All those areas (and potentially others) taken in combination, and the market for this device doesnt seem quite so small. Personally, I think it would be a good thing for Apple to get out in front of, provided they take the correct steps and do it the right way.
I'm not so sure the market will be that comprehensive. So far that market segment has not really embraced the tablet pc, there's no evidence to support that once apple produces one they'll drop their current set of tools and buy one of those.