Do vice-presidential picks matter? Almost, maybe, not really.
In 1988, George H.W. Bush picked Dan Quayle. After a couple gaffes, Quayle was easily portrayed as an idiot. Quayle's famous dress-down by Lloyd Bentsen at the vice-presidential debate meant that Bush won despite having Quayle as his VP pick, and even though presidents frequently changed vice-presidents over history (FDR did it more than once), Bush's loyalty to Quayle helped cost him the 92 election. (That, and Ross Perot, and the recession, and...)
In 1992, Bill Clinton was a womanizer who skewed too centrist in the primaries, so he balanced that by picking squeaky-clean liberal Al Gore as his running mate. Alas, Clinton only got 43% of the vote. Oh wait, Bush only got 39% and Perot got 17% of the vote. So he won.
In 1996, Bob Dole picked Jack Kemp. That made the GOP base excited for about a week. Then Clinton won 49% to 42%.
In 2000, in a personality year, Al Gore picked the moderate Joseph Lieberman. George W. Bush, the fratboy who was picked because brother Jeb hadn't been governor long enough, needed gravitas and picked Dick Cheney. People may forget how back then, lots of people saw the VP debate and wished the choices were instead Lieberman-Gore and Cheney-Bush. Both VP picks were good for the candidates, but unfortunately for Gore, many Florida voters didn't know how to fill out a voting ballot properly. Among other things.
In 2004, John Kerry picked John Edwards. I guess he felt like he had to, but Edwards didn't even deliver his home state of North Carolina, so it was a useless pick.
In 2008, despite Hillary Clinton's constant goalpost moving, Barack Obama will be the nominee, and he has to figure out what flaws he wants to cover with that VP pick. Does he need to appease the women who feel screwed by Hillary losing? I think making Hillary the VP would be a colossal mistake, and it would only happen if she wanted it. I don't see why she would. My guess is she'd want John McCain to win the election, and then she can run in 2012. If Obama wins, she can't run until 2016, by which time she'll be 69, a little too old. (Look at all the Methuselah jokes about 71-year-old John McCain.) So Obama could pick a female governor, but my guess he'll stick with a white guy. Because America isn't "ready" for Prez AND Vice-Prez to be historic. And it will probably be a Clinton supporter. Top two contender names I'm seeing floated would be Evan Bayh or Ed Rendell. Bayh, sure. Rendell would have to do an about-face on some opinions he's expressed the past few weeks.
Now if Hillary wanted to be VP, I imagine every soldier in the CLinton machine would pressure him to take her. If I was him, I wouldn't want her on the ticket because A.) who would want Bill & Hillary puttering around the White House?, and B.) that Clinton dead-pool list would make me nervous.
John McCain has a wide open field. He probably needs someone considered more conservative than him, and younger. Shouldn't be too hard. I don't think it'll be any of the men he faced in the primary. Rudy Giulani won one whole delegate. Fred Thompson looks as old as J-Mac. McCain and Romney got personal (McCain saying Romney doesn't stand for anything; Romney saying McCain's Nixonian tactics were "reprehensible"). Mike Huckabee's an absolute joke. For J-Mac, he will likely go with a governor, and the two most likely are Tim Pawlenty or Charlie Crist. (I've heard Utah's own Jon Huntsman Jr., but he's Mormon and moderate; a problem in some southern swing states).