The rest of you: wait a month
The tentatively exclusive Elder Scrolls V: Skyrim - Dawnguard DLC is now available on Xbox LIVE; hit the source to download away.The content is a little over 500MB in size, and costs 1600 Microsoft Points. To trigger the quest, all you need is a level 10 or above character, and you'll start hearing rumors of vampire hunters called "Dawnguard."
As you may recall with bitterness and angst, the DLC will launch one month from now on PC and PS3.
Dawnguard is heavy on vampires and presents you with the ultimatum of beating them or joining them.