I just posted on the front page about this. There is no beta. Some members of our community have taken it upon themselves to private message and e-mail members of our forums about this, but I'm afraid what they're saying isn't true.

The funny thing is that it's part of my job to answer e-mails from the public, so imagine my confusion when people started e-mailing in wanting to sign-up for a non-existent beta for Halo Wars. I even got to read all about it on Halopedia, before the admins over there took down the misinformation.