You will not be able to try Diablo 3 at Dreamhack winter 2009, only WoW: Cataclysm & Starcraft II source says:

Quote from: Ancilorn (Source)

We’re pleased to announce that Blizzard Entertainment will be attending Dreamhack Winter 2009, the world’s largest computer festival, which is taking place from 26th to 29th November in Jönköping, Sweden.

Dreamhack attendees will be able to visit the Blizzard booth to take part in various competitions and contests, and also to play both StarCraft II and World of Warcraft: Cataclysm.

You can find out more about Dreamhack Winter 2009 at http://www.dreamhack.se/dhw09.

We look forward to seeing you there!