The focus will be on the new Talent specialization and Mastery system changes introduced in Cataclysm.
The chat will be made through Twitter. All you need to do is follow @WarCraft.
To submit a question, use the #BlizzChat tag between 17:00 and 18:00 PDT (02:00 CET). All answers will be posted in a thread afterwards on Blizzard's World of WarCraft forums.
Links
WorldOfWarcraft.com - Source
WorldOfWarcraft.com - Read up on the changes