Update: A ton of concept images from the game have been posted on Flickr by "goneisgone". See them here.
Gama's reporting that Ensemble's cancelled game was a Halo MMO. From the piece:
Following the recent announcement that Microsoft-owned Age Of Empires creator Ensemble Studios would close after the completion of Halo Wars, Gamasutra has discovered that a now-canceled Halo MMO was in development at the studio, unearthing prototype UI and level screenshots of the Ensemble-developed project.
The prototype art, which was at one point made available on an Ensemble-linked online artist portfolio website, further confirms previous rumors that the studio was working on an MMO based on the Bungie-created sci-fi franchise.
There's a screen and some art through there. Seriously. Thanks, Jim!