I don't know whether it has been posted, but according to what I read http://www.jeuxvideo.com/articles/0001/00017154-the-elder-scrolls-v-skyrim-dawnguard-e3-2012-preview.htm (in French), the "Tyranny of the Sun" refers to a prophetized obliteration of the Vampires that the Vampires... well, fear, and want to avoid
Actually I am quite worried, for neither a total annihilation of Vampires nor a perpetual occultation of the Sun seem to be sustainable options for the next Elder Scrolls games and DLCs to come, for I suppose many players wouldn't like to be deprived either of vampirism or of daylight for entire games. How do you think they'll handle this issue, scenaristically, in the next games ? Dragon break ? Providential intervention of Molag Bal and Lamae Beolfag or of the Psijic Order
Spoiler (now owning the Eye of Magnus)
or Thalmor (with its new "Dawn Magic") , either to restore the Vampire or the Sun? It would look quite much like a deus ex machina, don't you think?
Edit: Just to make it clear : these slight worries don't prevent me at all to be thrilled and impatient to discover it all. I do think that Bethesda did a wondrous job on this DLC, which really looks amazing.