Yeah, old news to me (I made the first thread about dawnguard, like, one and a half month ago

). Still, while I am SURE that dawnguard represents *something* coming out from Bethesda/Zenimax in the near future, I cannot actually say with 100% certainty that it will be Skyrim DLC. The only clue we have now is a name. We have absolutely no idea what it will contain. The only way this is connected to Skyrim is the way the name sounds something very "Elder scrolls-like" and there are numerous references to 'dawn' in skyrim. Also, the lettering on the trademark page is similar to the one used in Skyrim. That's about it.
Note this this method was also how some people discovered the 'Skyrim' name 6 months before the official statement.
OK, since the thread warrants it: here's what I (wildly) speculate: the DLC will focus on the Thalmor and high elves who have made their mission to destroy the dominance of mankind in Tamriel and return the world to the Dawn era (i.e. "Before the Ages of Man"). There's enough foreshadowing in Skyrim that the next DLC (or the next elder scrolls title) will involve the Thalmor in a major way. There are also some factions that seem under-utilized in vanilla Skyrim, like the Vigilants of Stendarr.