Traditionally, the only question a presidential nominee would ask when looking for a running-mate is: Can they deliver their state. In truth, LBJ was (arguably) the only VP candidate who helped secure an election, when he delivered Texas for JFK.
George W. Bush almost learned this lesson the hard way by picking Dick Cheney (who could only deliver Wyoming). Of course, Bush lost the popular vote, and almost lost the election. With the election being so close, one wonders if another VP pick would have given Bush the extra votes he needed to win the popular vote.
But 2008 may be the year of the "Conservatie VP" when candidates like Romney, McCain, or Rudy decide the best way to turn-out-the-vote is to "balance the ticket" with a darling of the conservative movement. This could be good news for conservative candidates like Sen. Rick Santorum, Sen. Sam Brownback, or former Ohio Secretary of State Ken Blackwell.
Think of it; wouldn't you be so much more excited if you knew it would look like this:
McCain/Blackwell '08
Rudy/Santorum '08
Romney/Brownback '08
Vote for your favorite on the comments -- or mix-and-match ...
*** Update: 12:02 PM
Thanks to John Hall, I have just added 2 more options ...
Mitt/Newt '08
Rudy/Gilmore '08
... Cast your vote in the comments.