From the PlanetXbox360 contest:
"We just got an e-mail from Microsoft asking us if any of our readers would be interested in playing the Halo Wars beta a little early, well by early I mean a whole week! We have 15 codes up for grabs and all you have to do is leave a comment in this story (on the website) and/or join our Twitter page and then sending us an @reply asking you be entered into the Halo Wars Beta contest. The codes will be given away on Wednesday: January 28th, 2009 and will allow access to the demo starting on Thursday: January 29th, 2009."
Just incase there is some confusion you can enter by following their Twitter account and then sending them an @reply or by clicking the link out and posting on THEIR site, not in the N4G comment section. Thanks everyone: http://www.planetxbox360.com/article_5646/Halo_Wars_Code_Giveaway_-_Play_the_Demo_Early