Blizzard CM Bashiok has announced that the official Cataclysm start time for all North American servers will be Dec. 7 at midnight PST. This will include servers on other time zones. So, sorry eastern time people. You may be able to pick up your copies of the game at midnight EST, but you'll have to wait a few hours for the west coast to catch up before you play.
The full announcement is as follows:
World of Warcraft: Cataclysm will destroy Azeroth as we know it; nothing will be the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion (available Dec. 7, 2010), from brand new races to revamped quests and zones. Visit our Cataclysm news category for the most recent posts having to do with the Cataclysm expansion.