Barack Obama and Joe Biden are from solidly blue states. John McCain and Sarah Palin are from fairly solid red states (though some, myself included, would argue that medium- to long-term Democratic prospects in Arizona and Alaska are good).
Every four years when the veepstakes begin, there is much talk about how this or that candidate in consideration offers the potential benefit of putting his or her home state into play. Certainly Al Gore helped Bill Clinton nail down Tennessee in both 1992 and 1996, though, tragically, the future Oscar winner couldn't win his home state in 2000. Still, you really have to look back to 1960 for a truly powerful example of a candidate who helped pull a state into the win column for a ticket that otherwise would not have, and who helped tip the election.
I think it's fair to say that presidential candidates and their advisors know this. The tip-a-state factor just doesn't matter anymore. Partly, this is a function of how blue the blue states are and how red the red states are. (In 2000, a tight popular-vote contest, there were 28 states decided by 10 points or more; in that 1960 election, an equally tight popular-vote contest, there were just 14.) But I think the larger reason for reducing home-state flipping to a second-tier criterion is that the veep selection provides other political assets for the presidential candidate or is used to blunt criticism of that nominee.
Biden is an avuncular, hand-on-the-shoulder senior statesman who lends gravitas, assurance and policy substance to Obama's ticket, which is why he edged out somebody like Tim Kaine of swing state Virginia. Palin is younger, female, solidly antichoice and tough to criticize on labor issues because of her husband's background, which is why she edged out somebody like Rob Portman of swing state Ohio.
As John McCain might say, My friends, mark your calendars: The era of the home-state-flipper veep pick is dead.