Skyrim's first piece of DLC is named 'Dawnguard' and will arrive in summer for the US, which means around June to September. Only information right now is that the DLC will first hit on Xbox 360 and more info will be announced on E3, which is a month away.
Bethesda Blog teased the DLC with the image above and pretty much says nothing else in the post. I have Skyrim on the 360 so I'm definitely going to pick this up when it gets released. Yes even without info, more content for Skyrim is a definite win in my eyes. Don't you agree?
Don't worry about Xbox 360 getting first dibs on the DLC, its only a month head start. And if you ask me, one month of first access to DLC is nothing compared to what PC players get to enjoy - MODS. Playstation 3 get no love apparently.