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