Pete Hines has announced on twitter that "We have not announced Dawnguard for any other platform, nor given a timeline for any such news. If we have news, I promise I'd tell you."

That seems to be the best info we're going to get out of Bethesda. Looks like it's going to be a while before we see Dawnguard.

http://www.gamesas.com/topic/1398836-pcps3-release-date-part-5/