At least the first thirty days.
Dawnguard's mostly likely not going to release a month from now. When you consider that we don't have the latest patch, it's clear that Bethesda's a little behind on the playstation. That's not to bash the company. I've always heard the programming architecture for the PS3 is very complicated compared to the other big two consoles. I doubt we're going to get Dawnguard before we get that. At most? It'll be released at the same time.
Beyond that, previous DLC for Bethesda games on the PS3 has not been reliable released thirty days after the fact. When you look at New Vegas, Dead Money was released to the 360 on December 21st , 2010 and didn't see release on PS3 until Febuary 22nd of 2011. That's about two months. Beyond this, I think it's very telling that if you look at some of the staffers twitters they've repeatedly said they haven't announced a time for other platforms. That tells me that it's probably not going to be right after the microsoft exclusivity goes away.
So let's wait. Let's be patient and let's not log back on here a month from now angrily wondering where Dawnguard is. The idea that it'll be released a month from now on the dot is an idea that's come up in the community, but was never confirmed by anyone. I keep seeing it repeated as though it's a sure thing at this point and there's no reason to expect it to be, while there's at least a few admittedly vague reasons not to. It's more Skyrim and if you've been spending hours skulking through caves, cresting hills on your horse and committing dragon genocide you know it's going to be worth the wait.
Enjoy your DLC tomorrow Xbox'ers! I'll be eagerly anticipating joining you.