Here's the deal.. (at least the likely deal as we can learn from history).
MS paid for 60 exclusive days for Dead Money, the first DLC in Fallout: New Vegas. So, if we look at history, Dawnguard is probably 60 days. MS also payed Bethseda to shut up about promoting the PC and PS3 version for 60 or close to 60 days. After that period we will hear more.
IF Microsoft bought an indefinite exclusive deal, then Microsoft would want that fact promoted I imagine. The message being: "Don't hold out because it won't show up on your platform". But that message is not being communicated, so I think that it won't be forever excluded.
So wait 60 days for this DLC nonsesnse and you can play on PC/PS3. A small chance for 90 days, but I doubt it.
If MS payed for a 60 days exclusive deal. And Dawnguard was advertised with a 30 day exclusive period. THAT is false advertising. And I'm pretty sure that both companies aren't that thick headed.
I'm not into all the "the pc made you" crap. Because I first got into Elder Scrolls on the xbox. I bought skyrim for the xbox, and bought it again for the PC when I acquired enough savings for a good gaming PC. So I'm unbiased with my opinions.
I didn't buy dawnguard for the xbox, because I thought it was going to be a 30 day wait. And 30 days isn't too long. But Pete Hines has said that it hasn't even been announced for other platforms yet, so my first thought was that it wasn't going to be released on other platforms. Even though I think they will release it to other platforms, it definitely sounds like they might not.