There's been word going 'round that The Elder Scrolls V: Skyrim - Dawnguard was supposed to launch this week or thereabouts on PC and PS3, but it turns out this is not the case at all. In fact, Bethesda's VP of PR Pete Hines says the content isn't even announced for these platforms, at least not yet.
"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."
So, the DLC for these platforms is apparently in some state even unknown to Bethesda. That, or they're not allowed to say anything due to an exclusivity contract with Microsoft. Sit tight.