As some might remember, there was a post about Ensemble Studios' Halo MMO on this blog.
Nevertheless, I would like to add some information.
For one thing, the MMO has never left the pre-production status. Only a small team of not more than 8 people developed working prototypes and concepts with a big reliance on traditional RPG gameplay.
For another thing, what most people do not know, Ensemble Studios had several smaller teams for building prototypes during the final year of AoE3 development. One of the prototypes established the basis for Halo Wars. Another still mysterious prototype was a first person shooter for PC platform. It might have been the "major project"(cancelled in late 2007) Bruce Shelley talked about on the studio's own blog. Or was it that prototype which was later known under the title "Wrench"? Time might shed some light on these things.
However, here is an early animation demo from some ES prototype which evolved into Halo Wars.
Additionally, here you see some inspirational pieces of concept art for the MMO(RPG).