Blizzard Entertainment will have a few representatives answering questions to PAX 2010 attendees. World of Warcraft: Cataclysm and Starcraft II: Wings of Liberty will be playable at the NVIDIA booth.
Sadly, the Diablo III demo won't be showcased at PAX, but if you are heading to BlizzCon at least you will have a more updated demo version than the one shown in previous events.
PAX 2010 kicks off through September 3-5 in Downtown Seattle, Washington. The uber bad news for you is if you didn't buy a PAX ticket, don't even waste your time walking to the front door because ALL tickets were sold out on August 28. There won't be ticket sales at the front door.
Some of the PAX 2010 locations have been moved to a different place, so check out the official PAX website and read the 08/04/2010 update for instructions.