Credible source Gamasutra reports that Ensemble Studios, who are now in the process of closing down, were working on a massively multiplayer Halo game until last year. They have unearthed an prototype screen with user interface along with some artwork.
Rumours of a Halo MMO has been circulating for a long time, and of course just because the project at Ensemble Studios was cancelled it doesn't mean there is no possibility of Halo Online in the future.
"Thus, it appears that the Halo MMO project was likely internally cancelled some time in mid-late 2007, without any formal announcement from Microsoft that it was ever in development. The below UI mockup and concept/level screenshots also date from the 2006-2007 period."
Here is the artwork...