Bethesda really likes the way the beta patches have been working on the PC for Skyrim. Because of this, they've decided to extend this over to the Xbox 360 and have opened up beta signups for the new Dawnguard expansion for 360 owners. Just head over to The Elder Scrolls' website and start answering questions about your playtime and character history. Just a friendly reminder, if you have a Bethesda forum account, they'd prefer you to use your email associated with that.
It's nice to see Bethesda seriously investing in betas for patches lately, especially with some of the earlier patches for Skyrim making the game even more buggy (not to mention their personal history of shipping notoriously buggy games). If you don't care at all about helping make the game more polished, did I mention that if you get into the beta you get a free download code for the expansion? So, you should definitely go sign up for the Dawnguard beta ASAP. I'm sure spots are filling up quickly.
E3: Apply for the Dawnguard Beta on Xbox 360 [Bethesda Blog]