I realize that the Soul Cairn of Oblivion is essential to the vampire lord storyline of the new dlc and the entrance to it is in Castle Volkihar, but I am wondering if members of of the Dawnguard will also be able to visit there. Much of the new unique content, such as the soul tear shout and the summoned daedric horse, are only able to be found if you go into the Soul Cairn, yet the game footage that I've seen shows a character with all Dawnguard armor and a crossbow riding the summoned horse. It could be that the final battle with Harken will take place it the Soul Cairn or something like that.

Check the gamespot interview with Matt from Bethesda (on YouTube somewhere)...Matt confirmed that a Dawnguard aligned character would indeed visit Soul Cairn.

We must be able to.

Yes, everyone has access to it.

Obviously yes because you go there to rescue the vampire lords daughter and in the trailer he asks you what side you are on after you rescue her