Fra Bethesda:
Want a chance to play Dawnguard early? Following the success of our PC beta updates, we're excited to announce our Dawnguard Beta for Xbox 360.
Visit beta.elderscrolls.com to apply. The process should only take a couple minutes, but make sure to fill out all the information accurately. Applicants need a registered account on our official forums and you'll want to make sure your forum email address is updated.
We will be selecting applicants in about a week. Players accepted into the beta will receive an Xbox LIVE code to redeem Dawnguard. The beta is the full version (complete with Xbox LIVE achievements) and those participating will not have to purchase the content.
Thanks again for your continued support of Skyrim!
Link:
http://dawnguardbeta.elderscrolls.com/form.php