It was just announced at CES that Halo Wars will be available on February 28th (see below with world wide dates) with a demo hitting Xbox LIVE on February 5th.
Edit: There has be an update to the Halo Wars release date. In addition, I have details on specific regions:
Japan and Asia Pacific: February 26
Europe: February 27
North America: March 3
Note: The demo is still on track for a February 5th release.