Although Bethesda has only announced Dawnguard for Xbox 360 so far, previous so-called "console exclusive" downloadable content from the publisher has made its way to other platforms on future dates. In other words, it could just be lagging on the PlayStation 3.
What, do they not remember reporting that the Skyrim DLC was a timed exclusive for the 360? Of course it will make it to PS3. They never said it was console exclusive.