I have heard of some tablet PCs having Wacom pressure sensitivity built into them, but the pressure sensitivity has never exceeded 256 levels so far. I hypothesize that the technology required for more than 256 levels is still too big/heavy to fit sensibly into a tablet PC, or perhaps Wacom is not yet willing to give up the patent rights for their manufacture.
I have also heard some artists say that 256 levels is enough, and they happily use Zbrush and other 3D sculpting software on their tablet PCs. Personally I feel that 1024 levels is the minimum to get decent results.
Despite the existence of tablet PCs with pressure sensitivity, the 3D processing power of desktops still greatly exceeds tablet PCs, and because this kind of processing power is so important to 3D artists, a desktop with a standalone Cintiq is still the best choice in the end.
Desktop PC's will always exceed portable devices in all areas but one, portability. This is my only reason for being interested in them. I'm sure some day they'll be as powerful as the desktop PC I'm currently using right now though, and at the rate things seem to be advancing I'm willing to bet it won't be too long of a wait.
About pressure sensitivity, I was thinking about this the other day after commenting on the Bamboo. Specifically how they have the same pressure sensitivity as the old Intuos3's did (1024). It got me thinking that this may be nothing more than a marketing tactic, similar to the megapixel number on digital cameras. 2048 is double 1024, so it must be twice as good, right? The thing is the Graphire, with it's 512 levels of pressure sensitivity, never felt much different to me than the Intuos2 and Intuos3 did. Ditto going from them to an Intuos4. It would be interesting to study whether the difference some people feel is just a placebo effect or not.
PS: You're correct about it being a patent issue (with Wacom's electromagnetic induction technology). Their patent, US4878553, supposedly ran out around the end of last year. Hopefully this means we'll see a corresponding change in tablet PC's as a result now that other manufacturers are no longer limited by this. I could be wrong though. Looking at Wacom's patent record, I get the impression they keep applying for new patents using the exact same technology and are just rewording it slightly. Wouldn't surprise me in the least considering their near monopoly on good input tablets. I'd want to protect that too.