Project Titan is the name given to Blizzard's next MMO. Despite having very little information to go on, we do know that Titan is being developed as a successor to World of Warcraft in terms of its dominance over the MMO market.
Blizzard plans to give it their all after they release Diablo III, and the Mists of Pandaria expansion for World of Warcraft, which is tentatively dated for a late 2012 release.
With no BlizzCon this year, we can only expect Blizzard to make an announcement about Project Titan in 2013, and with any luck, the game will be out by 2014. Granted, given Blizzard's predilection towards malleable release schedules, the game will only be "done when it's done."
Blizzard's Rob Pardo let slip on Twitter that Titan is currently playable despite being in a 'pretty rough' state.
Needless to say, we won't be hearing much about the game's development until after Diablo III is released on May 15. Assuming it's released, and they don't allow the deadline to slip again.