Bungie community and PR director Brian Jarrard recently told gamesindustry.biz that a Halo MMO has been the subject of "heated discussions" at the house that built Master Chief. Jarrard opines on the topic for a while, concluding that a lot of new lore and characters on top of the already-extensive universe that is Halo for a workable MMO.
That point of view rings quite true with us here at Massively. Typically, any time a non-MMO property is tapped for entry into the genre there are mountains of conceptual alterations that need to be made -- not to mention the catch 22 Jedi concession. You can't have a compelling Star Wars MMO without Jedi, however there needs to be some serious lore considerations if a developer is to allow hundreds of players run around their game world as laser-sword swinging, neo-space faring, monk/samurai. The same rule applies to the rare and elite Spartan warriors of the Halo universe.