The Bethesda Blog has word that The Elder Scrolls V: Skyrim - Dawnguard is now available on Steam, offering Windows users this DLC for the RPG sequel. This update for PS3 owners explains that version of the DLC is being held back because of dissatisfaction with its performance on that platform. Here's word on the pack:

Dawnguard™ is the first official game add-on for The Elder Scrolls V: Skyrim® - the 2011 Game of the Year. The Vampire Lord Harkon has returned to power. By using the Elder Scrolls, he seeks to do the unthinkable - to end the sun itself. Will you join the ancient order of the Dawnguard and stop him? Or will you become a Vampire Lord yourself? Featuring an all new faction questline and locations, the ultimate choice will be yours.