The official trailer for Skyrim's Xbox 360 exclusive episode Dawnguard is vamptastic.
Dawnguard actually delivers a lot more content into The Elder Scrolls: Skyrim. There aren't a ton of specifics as of yet on the add-on, but it looks from the trailer like you can cast yourself as either vampire, or Dawnguard (vampire hunter). Particularly for fans of the undead, this looks like one that definitely won't suck. Ahem.
The Elder Scrolls: Skyrim Dawnguard will be available for Xbox Live this Summer, it'll set you back a cool 1200mspts.