Thisimageessentially sums up all that we know about Dawnguard. Soak it in. It's all you're getting until E3 rolls around next month. Well, that's not exactly true. We do know a few things, but none of them have been confirmed by Bethesda.
Last week, a user took a special peek into the latest patch's source code and found a few hints about what the next DLC pack might include. In a file marked "DLC01" he found references to "snow elves" as well as a "Snow Elf Prince" and crossbows.
It's been interesting to watch how Bethesda has handled this DLC announcement. They're doling out the tidbits piecemeal, and are plotting a major reveal at E3 2012. All of which suggests to us that this is going to be a pretty giant DLC pack in line with the Shivering Isles expansion for Oblivion.
With huge sales for Skyrim and 8 months to work on this DLC, we're expecting big things from Bethesda.