Skyrim's Dragonborn DLC arrives tomorrow, December 4, 2012, but you won't have to wait that long for information about the expansion's return trip to the island of Solstheim, last seen as an expansion for Morrowind. An alleged Dragonborn beta tester revealed the major facts about the content pack over at TheOuthousers (via CVG).
Be warned before you follow the source link: there are lots of spoilers to be found (assuming the info is true). You might simply prefer to wait for the DLC's launch tomorrow and find out for yourself. I will say one thing though: there's (again, supposedly) an Achievement for taming and riding five dragons. FIVE.