Official Blizzard Quote:
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.
The demo is the same from previous expositions, but no less fun!