Bethesda finally reveals Dawnguard as Skyrim's first DLC. It was teased awhile back with the developers showing one image teasing fans of the upcoming DLC. Take a look at the trailer for the new content.
Dawnguard seems to give the player a choice to either side with the Dawnguards or the vampires. The content also adds new features to the game. Like new vampire forms, crossbows, and different mounts. Mounted combat is shown in the trailer but is not a Dawnguard feature I believe. The patch for this feature has not been released but the patch is out in Steam for players to beta test the patch changes.
Dawnguard at the moment is listed for this summer(Around June to September) for the Xbox360 for 1600 Microsoft Points. No word yet when PS3 and PC will get the DLC. So Xbox 360 might get the content exclusive for a month just like what they did with Fallout: New Vegas' added content - Being released in Xbox Live 1 month earlier than the rest.
[Source: Bethesda Blog]