Bethesda has announced the first downloadable expansion to The Elder Scrolls V: Skyrim. It'll be called Dawnguard and it's scheduled to come to Xbox Live this Summer. No other platforms have been announced at this time, although Bethesda's usual M.O. is to release on PS3 and PC after an exclusive time period.
So far, that's all that has been revealed, with Bethesda promising more details at E3 in a month's time.
This is a thing!