[Click for full image] UPDATE: Full Flickr set here.
Gamasutra have unearthed evidence that the Ensemble Studios project that was cancelled during 2007 was in fact a Halo MMO. The evidence includes several pieces of prototype art, with one mocked up screen (above).
The title seems to have been in development in 2006 and some of 2007 -- though it's by no means clear that it was the only MMO-related title in development at the studio at that time.
What is clear, however, is that Ensemble's Bruce Shelley mentioned in a June 2008 blog post that they "...set up three prototype teams out of the staff of a major project that we cancelled. After six months of very interesting work, we have now stopped two of those prototypes, with one getting more time to demonstrate the value of its concept."
Seems I was right about a Halo MMO being in development, but my timing sucks... It also means that a Halo MMO could well now be in development at another studio. Let's try and guess!