http://uk.games.ign.com/articles/122/1221479p1.html
Bethesda parent company Zenimax Media has submitted a trademark application for new downloadable game content. In a filing submitted last month, Zenimax refers to a project called 'Dawnguard' that's specifically referred to as "computer game software for use with computers and video game consoles" and "downloadable computer game software offered via the internet and wireless devices."
The filing doesn't link the project to any specific franchise, but given Bethesda's recent ideas about Skyrim DLC and the specific mention of both consoles and PC, a connection to that series is certainly possible. In the past, director Todd Howard commented that Skyrim DLC releases "don't have a timetable" and that "they'll have a lot of meat on them."
Outside of Skyrim, Bethesda has also recently been hiring for a next-gen project and Zenimax Online is rumored to be working on an Elder Scrolls MMO. For all we know, either of those projects could also be connected to the trademark filing. In the PC gaming world, even full games are technically "downloadable," so Dawnguard could just as easily be an entirely new game.