The first Skyrim expansion, Dawnguard, has been detailed a bit more in an official trailer from Bethesda today.
In the trailer, we hear narration from a new character, who appears to be a vampire lord of some sort. From the trailer, it appears you will be able to take your Dovakhiin down either the path of light, joining the Dawnguard to put down the vampire threat, or join the vampires themselves, possibly sprouting wings and getting access to a glowing, skeletal warhorse.
The dark side is sounding like the better end of the bargain, if this trailer is any indication.