This afternoon I got to spend about 15 minutes playing with a tablet PC from Acer  (TravelMate 100) and one from Compaq (TC1000C).  I hefted them wrote on them, and played with some of the apps.  My first impressions:

• I think I'd like one of these.  They have a very nice form factor that is more usable than a notebook in a meeting, airplane, etc. 
• I liked that COMPAQ keyboard detached.  It wasn't a lot lighter, but it was much thinner. 
• The Acer was VERY hot on the bottom.  It would cook your legs if you had to sit with it on your lap.  The COMPAQ was nice and cool (they'd been on for about the same length of time)
• They're pretty expensive for something which I don't see replacing my laptop.  I'd like one, but not as my only notebook.  I'd give up my iPAQ for one though.