In a session called "I Shot You First: Networking the Gameplay of Halo: Reach", it has been revealed that Bungie's next project is indeed an MMO, expanding to the MMOA genre. It won't be World of Warcraft in space though, as was previously suggested.
The aim for Bungie is to have a world that's always there with you, a notion that was tease at last year's Game Developers Conference by Bungie's Joseph Staten.
Nothing else was revealed about the title and we know it's not planned for a 2011 release. Hopefully we'll get to see something about the title at this year's E3 though.