I don't think anyone is surprised that Skyrim's next DLC, Dragonborn, will be releasing on 360 considerably ahead of PC. Which is to say, tomorrow. At least we've been given a general ballpark this time: the quote is "early next year" for the PC release, which could mean as soon as January or as late as maybe March. Assuming a 30-day exclusivity contract with Xbox and a similar delay to what we saw with previous DLC, we're probably looking at the first or second week of January.
In the meantime, you can check out the official trailer for Dragonborn and listen to this Elder Scrolls loremaster dissecting every scrap of it to hypothesize what it's probably going to be about.
The official announcement can be found on Bethesda's blog .