Okay, so, Bethesda's parent company, Zenimax Media, filed a Trademark request for the name "Dawnguard". see here). Its possible that this may be used for Elder Scrolls. Then again, it may also be used for something else entirely.
After all, Zenimax Media is a big company, and Bethesda is not their only company. They also own id Software (Doom, Quake, amongst others); Arkane Studios (currently working on a game called "Dishonored"); and Tango Gameworks (Resident Evil, Devil May Cry, Final Fantasy).
So it is entirely possible that this was for one of the many games out of one of those companies. Or, it may be DLC to do with the Mythic Dawn, which appeared in Skyrim already in a lesser role. We're just going to have to wait and see, really. Oh, and if you're wondering where that list of companies and games came from, it came from Zenimax's Website. Enjoy. :)
So what is it?
Well, as posted in the first link, the Dawnguard patent is for a goods/service, that includes "Computer game software for use with computers and video game consoles; downloadable computer game software offered via the internet and wireless devices". In plan english? It looks like it WILL be DLC content, but, this is not confirmed by Bethesda just yet. Don't be surprised if this is one of the dlc's available, though.