In an interview with Strategy Informer over World of Warcraft: Cataclysm due to go up later today, Game Designer David Kosak remarked that the company had yet to say anything official on the title.
When further pressed, Kosak's co-worker, Greg Street, had this to say:
"Well, the World of Warcraft team is busy on World of Warcraft, and we have an RTS team but they are busy with Starcraft II, we don't right now have enough bandwidth to tackle something like that."
This would suggest that the game isn't in development at all, although Blizzard have always been very shrewd when it comes to information on their games.
Stay tuned for the full interview.