Ok, you have to be kidding me. Supposedly this is nothing more than an outlandishly freak coincidence, but according to the developer blog of Ensemble's Bruce Shelley, the internal codename used for the Halo Wars project is Phoenix-- the same codename used for a scuttled project at Bungie a few years ago that was rumored to be a strategy game of some kind, perhaps a fantasy game in the vein of Myth.

Shelley's entry doesn't give a lot of information about the game, but it does tell us who at Ensemble is working on it so far:

The Halo Wars team is the fastest growing at ES right now. Much of The WarChiefs team is rolling over to it and many of our new employees are also joining. The team is being led by veteran ES producer Chris Rippy and Lead Programmer Angelo Laudon. Both have been with ES from the start, nearly 12 years ago. Angelo was Lead Programmer on Age I and Age II. The Art Lead is Mathew Goldman, an industry veteran with many titles to his credit from years at Bioware. Lead Designer is Graeme Devine, another industry veteran with 20+ years experience. We think the project is in great hands.

The name Graeme Devine at least should be familiar to those fans perhaps new to the RTS genre but familiar with first-person shooters, as he worked with Id Software on Quake 3 and Doom 3.