Let's look at the technologies that are fairly common on the PC side, and that Apple has yet to implement in its portables:
USB 2.0
Built-in BlueTooth (or an internally-held expansion card)
DDR Memory
Hardware-based variable bus timing
These have been discussed at length in these forums and I think are the most likely candidates for the next "major" revision to the portable lineup.
The tablet PC phoenomenon is interesting. Microsoft only expects a few hundred thousand units to be sold in its first year, which, to them, represents more of a test market than a viable business unit at this point in time. Apple, however, is a niche player in the industry and such a technology, if aggressively developed by the company, could represent a potential opportunity for the company to capture market share. A hundred thousand units to Apple would be a nice additional revenue stream. So, while I still don't think it is likely they will come out with a tablet any time soon, this is one argument supporting that possibility. Of course, being the market follower and having to overcome Microsoft's marketing efforts to link its brand with the tablet PC concept would mean Apple would have to spend BIG money on both R&D and promotion. That's money I don't think they have. So, personally, I think Apple will steer clear of the tablet PC market and introduce either an alternative, or invent a whole new product category that hasn't yet been seen on the market.