The 30-day exclusivity deal MC had with Bethesda has recently ended but Pete Hines has announced there isn't even a planned date to announce that Dawnguard wil be coming to PS3 or PC

Source: http://www.escapistmagazine.com/news/view/118752-Bethesda-Has-No-Timeline-for-Dawnguard-on-PC-and-PS3