Just wondering if anyone knows if Dawnguard is/will be available in a boxed copy or will it be pure digital download only?

No plans for Dawnguard released in any other way than digital as of now. It will probably be available with the release of the Skyrim GOTY edition some time down the road.

Yeah Bethesda doesn't usually release DLC in any other form besides download until the game of the year edition comes out with all of the DLC included. That won't come out for a year or two.