Obama just eliminated a few of McCain's options.
Consider "family values". So far I haven't seen a hint of scandal in this area from either Obama or Biden.
McCain, on the other hand, is constantly having to apologize for past behavior. Imagine the Religious Right's dilemma if he chose Guiliani, for instance. He could choose Huckabee, but how many people do not want a Southern Baptist minister as president? And it's entirely possible McCain wouldn't live out his term.
Romney would be an option, excepting Romney and McCain barely pretend to tolerate each other.
I think Obama's choice of Biden has caused McCain some small problems.
Arf