Gamasutra has broken the news that Ensemble Studios, responsible for Halo Wars, was working on a Halo MMO. It's not clear at what stage the project was canned, but Gamasutra's analysis based on available material is that it was probably cancelled in mid to late 2007.
Gamasutra has also gained access to some prototype UI and level screenshots. With the help of our own lead blogger here at Massively, Michael Zenke, they've attempted to unscramble how the UI might function. Michael points out the clear similarities with the World of Warcraft interface, identifying what appear to be a mana bar, inventory button, talent tree and other functions. Louis Wu of the unofficial Halo website halo.bungie.org offers expert commentary on the level shots, identifying the familiar and the hitherto unseen aspects.
As Gamasutra points out in the article, there are multiple development teams at work on Halo-related projects right now. It's therefore possible that a Halo MMO could still exist, but if it does, there's not a whisper of its presence so far.
Update: Apologies for referring to Ensemble Studios as 'closed'; they are scheduled to be closed after the release of Halo Wars, but this has not yet taken place.
Update 2: Gamespot's Rumor Control department has taken a long hard look at the evidence, and has pointed to a Flickr photostream containing 50 images of what appears to be a Halo MMO in development.