I've seen pictures for years about the tablet computers, the latest about an Apple one...with various things saying they would be the next big thing.
Why they haven't, and won't be anything more than a niche:
Every time I can remember seeing a marketing picture of one being used, it shows some document with red hand-drawn circles and arrows and handwriting on it. "Fix This." "What About Europe."
For the CEO and high up management types, that looks awesome...but they're the ones drawing the red marks (and approving Tablet PCs). The people whose job is to actually do the "Fix That" is going to want a keyboard. I can only think of specialized jobs that interact better with a computer with a pen than a keyboard. Such as an artist or, say, a CEO who just has to mark up a document :)