okay...at the end of the trailer there was this red thing above dragonsreach...now i think that u enter dawnguard from whiterun...im not really sure if im correct but it does look like you do enter dawnguard from whiterun
Dawnguard is a faction, not a place for you to enter.
What you see is the sun being hidden by some force, that's why the unnamed vampire in the trailer says at that exact time that the tyranny of the sun will end.
that was the sun, altered with magic to not harm vampires.
It was a "Nightshade" effect. It prevents Vampires and other sun-sensitive undead from weakening or burning in the daylight. It's concept has been used in quite a few movies/shows/games.
I wonder if the sun will be like that all the way through. If it is you might have to fight the Vampires all over Skyrim. The possibilities it presents lol.
If the sun is blocked out at whiterun, i can imagine a scene with people running around like crazy while being hunted from vampires crowding the city. It would awesome to see so many vampires come out from hideing, you never know who might be a vampire in hiding. Belethor could be a vampire!?