Bethesda's official high-res DLC has a conflict with Dawnguard that results in Castle Volkihar's LOD looking like this:

http://3424.bugzilla.darkcreations.org/attachment.cgi?id=37

The cause is load order. As an ESM, Dawnguard will always load before ESP files, and the files the HD DLC provides are ESP files. Resource files are loaded out of the BSAs in the same order as your mod load order, so that means with normal usage, the HD DLC's distant LOD normal map will always supersede Dawnguard's resulting in broken LOD textures for Castle Volkihar.

You can fix this by instead registering the High Res Texture Pack BSAs in Skyrim.INI, as registered BSAs will always be superceded by BSAs that are loaded alongside data files.