According to worldofwar.net, a Blizzard community manager has commented regarding the unannounced MMO in the works at Blizzard.

The Unannounced MMO in development by Blizzard Entertainment has been confirmed by a community manager to be a new IP (Intelectual Property) that is not based on StarCraft, WarCraft nor Diablo.

I've heard that they are focusing on other projects. If this is true you can expect more of this, and less customer service/content. At this point they can just sit back and add some new arena/pvp gear every4-6 months and that's it.

Zarhym: This a common misconception. Such a misconception requires nothing but assumptions and misinformation to hold water. We have completely independent art and development teams for each project on which Blizzard Entertainment is working. Sure, people switch positions and may move to other projects from time to time. This is common practice in any company, whether or not the company develops games. But if a position is vacated, it is filled. You may have seen earlier this year that Jeff Kaplan, former game director for World of Warcraft, announced he would be leaving his position to begin work on the new, unannounced MMO. Tom Chilton stepped up to fill his role on the World of Warcraft development team. We're never short on talent and our teams are constantly growing. There's absolutely no reason or evidence to support the theory that we've shifted focus away from the Warcraft franchise. I'm surrounded by employees in this building working hard every day that can attest to this, were they not too busy working on a prioritized list of new content for this game to speak directly.

People are saying that there is another MMO in the works at Blizzard. Seriously, if there was, they would be advertising it already.

Zarhym: Games take years to develop from concept to release. Diablo III was in the works for years before being announced. StarCraft II was announced two years ago and isn't even in Beta yet.

We have nothing to advertise for the new MMO because it's a shell of a game thus far. We've already stated it'll be a brand new franchise, which means the lore, art, and game play are being developed entirely from scratch. It's an overwhelming process, but a process through which we excel. Our track record supports this.
In short: a new MMO is indeed in the works, with an entirely new IP, but don't expect to see it on shelves for years.

In short: a new MMO is indeed in the works, with an entirely new IP, but don't expect to see it on shelves for years.