As it was speculated on March 23rd, 2012 (
story), the name of the first DLC (downloadable content) for
The Elder Scrolls V: Skyrim is
Dawnguard.
Bethesda Softworks announced that this DLC is being developed for Xbox 360 - no word on PC and PS3 versions yet.
Dawnguard is scheduled for this summer - details will be unveiled at E3 2012.