Dragonborn appears to be the next expansion pack/DLC for Skyrim, according to modders who've accessed the files in the latest update for the game on the PC.
The files, which were mined by fans of the game on the official Elder Scrolls forum, make repeat references to "Dragonborn" and "DLC2" in the file names.
Here's a sample:
data\interface\Translate_english.txt
$Crafting_$DLC2ArmorBonemold BONEMOLD
$Crafting_$DLC2ArmorChitin CHITIN
$Crafting_$DLC2ArmorNordic NORDIC
$Crafting_$DLC2ArmorStalhrim STALHRIM
$Dragonborn Quests Completed Dragonborn Quests Completed
$DRAGONBORN_ESMName Dragonborn
$DOWNLOADABLE CONTENT_PS3 DOWNLOADABLE CONTENT
Locations in the DLC are also referenced, and include Castle Karstaag, "To Solstheim", "To Skyrim", Telvanni Tower, Standing Stones, Raven Rock, and Miraak Temple.
Judging from the names, the DLC is presumably set to take place in Solstheim, an island northwest of Vvardenfell (Morrowind) and east of Skyrim.
In addition to the location references, the files unearthed also reveal the presence of a Dragon mount with animations for all possible equipment types.
At this point in time, Bethesda has yet to make any official announcements about the Dragonborn DLC. Expect that to change in the near future.