File this in the rumour bin: a leak of Blizzard's product roadmap, with some analysis here. It may or may not be true, but it looks plausible.

But the big news is the last item, Titan, releasing Q4 2013. Could this be Blizzard's long-rumoured secret new MMO? We know they've been hiring for it, we know many of the original WoW developers were pulled off of Warcraft right after the Burning Crusade lanuch to work on something secret and new. But that's all we know.

But now we have a name. Project Titan. I've found a possible confirmation: Google delving turns up a guy named theNoid who claims to have an inside source at Blizzard. He's been talking about "Project Titan" at Blizzard for the last 18 months.

I can't go into too much detail, but a guy on my team lives with a code monkey who's working on Project Titan at Blizzard. This is their rumored successor to WoW, and is located in the Diablo wing of the building, higher security locked doors. --theNoid Jul 2009

Project Titan is locked tight at Blizzard, my friend lives with a guy working in there. This is Blizzards next MMO, which has been stated 1000 times to be a complete new IP and it is not WoW2. --theNoid Aug 2009

Project Titan? No, its more like 2013.. trust me. The core framework is all thats done, the engine. The programmers were tasked at building an engine for a game that did not yet exist and still does not exist. No art, no modeling, no nothing has started on Project Titan yet. Also, Blizzard has announced it being a new IP, which isn't entirely true either. Its a spin off of an existing franchise, a franchise that I think is going to excite people very, very much. Which one? Which would would people want more than any other? Blizzard runs a pretty tight ship, leaks are few and far between but tend to go unnoticed when coming from the unexpected --theNoid, Mar 2010

I also found another mention of the "Project Titan" name in September 2010 from a different source, but no idea if he's independent:

Next-Gen MMO (aka. Project Titan): Highly unlikely anything about this project will be published for a while. Not until WoW is on a decline. Anything else would just be self-sabotage. --Crowe

Taken separately, both the leaked product roadmap and the forum posts from "theNoid" are plausible but not particularly trustable. But the fact these two independent rumours confirm each other is quite interesting.

The name "Titan" for a Blizzard MMO is rich irony, btw, since Titan was also the name of the cancelled Halo MMO. Particularly if Blizzard's new thing ends up being an MMOFPS, as is rumoured. Who knows, maybe someone worked a deal and it's actually some of the same people? It seems very unlikely Blizzard would release a game based on the Halo IP.