Bethesda admits it's encountered a 'problem'
"Skyrim is a massive and dynamic game that requires a lot of resources, and things get much more complex when you're talking about sizable content like Dawnguard," wrote the studio. "We have tried a number of things, but none of them solve the issue enough to make Dawnguard good for everyone.
"The PS3 is a powerful system, and we're working hard to deliver the content you guys want. Dawnguard is obviously not the only DLC we’ve been working on either, so the issues of adding content get even more complicated. This is not a problem we’re positive we can solve, but we are working together with Sony to try to bring you this content."
I love Bethesda a lot, but if you really can't develop games properly for a system, don't release them on the system. Whether it's the fault of Bethesda or the way the PS3 works, it's clear by now that the studio's games are woefully inferior on that particular platform, and knowingly selling a poor product is worse than not selling it at all.