We've already seen the
teaser and today Microsoft officially announced
Halo Recon, a new stand-alone expansion pack that has you picking up the role of an ODST trooper. The game will dove tail in with Halo 3 as the game will fill in the gaps before Master Chief returned to Earth. Microsoft has released more breathless detail
online if you want more information on the game.
It looks like Microsoft will be keeping up the release of Halo and Gears of War games in alternating years. I'm not sure if this is great brand strategy or just another sign of how barren the Microsoft IP case is.