It appears that there seems to be no limit or restrictions as to when Bethesda can announce their latest DLC for the PS3 and PC.
Microsoft had signed a deal with the company that secured exclusivity over the download only content (and announcements with regards to other platforms) for a short period but it seems as if that period is over after a recent Twitter post.
It was spotted that a Bethesda employee said, "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."
The first two add-on packs for the RPG will be available exclusively on the 360 at least thirty days before any other platform.
It is baffling though as to why Microsoft didn't secure such a deal with the PC content too in order to completely dominate the marketplace for a short period.