Although Amazon has pinned World of Warcraft: Cataclysm expansion to be released next January but by a combination of patch info and official forum posts the date works out to be December 7th.
The first note is that World of Warcraft: Cataclysm beta patch tells us that the next arena season will start on December 14th. Combine this with a recent official blue post on WoW forums says that the next arena season will start a week after Cataclysm release. This indicates that it is to be released on December 7th.
Although this is still a rumour and Blizzard have said “We haven’t announced a release date for World of Warcraft: Cataclysm yet and we do not comment on rumours and speculation.” But it does look like it gaiz.