Microsoft Game Studios announced Halo Wars, an all-new real-time strategy game based on the legendary Halo® universe and built exclusively for Xbox 360™ by Ensemble Studios, creators of the Age of Empires series.

http://www.xbox.com/en-US/community/news/events/x06/halowarscomingtoxbox360.htm

You've got to see the trailer on the site in WMV-HD 720p.  Personally I think it hints at the style we should expect from the upcoming Halo major motion picture.  I'm disappointed there isn't a PC version as well though. I also wonder if there will be a tie-in with Halo 3?  Perhaps a command and control component via Halo Wars, directing troops (real players) in Halo 3?  That could be very, very cool.