We may have a release date now for Cataclysm thanks to sites like MMO Champion. By data mining the beta client, they found a notice stating that the new Arena Season will begin on December 14th. Why does this mean that Cataclysm will come out on December 7th? Because Blizzard said that Season 9 of Arena will begin a week after Cataclysm’s launch:
The next Arena Season will begin for level-85 players approximately one week after the official launch of World of Warcraft: Cataclysm.
So unless Blizzard changes the date of Arena Season 9, it’s a pretty safe bet to assume that you’ll be playing Cataclysm the first week of December.