Bashiok has confirmed that the demo of Diablo III from Blizzcon 2009 will be playable at the Warcraft Regional Finals held in Austin, Texas on August 21-22:
Of note "and the Diablo III demo seen at BlizzCon 2009." Which really shouldn't dissuade anyone if they're in the area, it's obviously worth playing even if you have before (IMO). I managed to clear the entire demo and it took about an hour and a half, so there's plenty to explore. That said I'm sure you won't be allowed to sit on the machines for that long in one sitting.
I haven't been to a Warcraft regional since 2007 (I think), when it was in San Diego. It wasn't super packed or anything, but a fun time and good matches. I'm not sure any more details on this event beyond what's on the website. What specific questions do you have? I can try to find answers.
