The World of Warcraft Community Site Under Development Page has been updated (thanks MMORPG Heaven), and likewise, The World of Warcraft Battleplan on the WoW General Discussion Forums (thanks Ant) is written by Blizzard producer Shane Dabiri on future plans for the MMORPG. The update includes vague reference to plans for an add-on, an inevitability for a game that boasts more than two million subscribers (story). Here's word:

Of course, on top of all this, each content update will also bring new quests, items, professions recipes, and continued class improvements. Some have asked about an expansion and what it might hold. I wanted to let players know that it is in the works, and we’ll release details as soon as we can.