Why though? The only reason dawnguard isn't going to be out in a week for PC and PS3 is because of the contract with Microsoft, once that ends, there is, again, no reason to hold back on us, especially because the game is already made and copies are just sitting there
The point of the contract is to get people to buy an XBox 360. 30 days isn't long enough, people like you and me will wait and just spend $20 dollars on it for PC. From what I understand it's only been made for XBox 360. After 30 days they can begin developing for PC & PS3 if they feel their is a market for it.
I wouldn't expect DG for the other platforms for quite awhile. If it was 30 days then Todd would have said "PC and PS3 version will be released 30 days after we release for XBox 360." However, he stated he didn't know when it would be released.