According to CNN’s Candy Crowley, Joe Biden made an interesting point when asked if he would serve as Hillary Clinton’s vice presidential nominee (mp3 from CNN’s Race to ’08 podcast):

DICK ULIANO [CNN]: [W]ould [Biden] accept a vice presidential slot, say, on a Hillary Clinton ticket or Barack Obama?

CROWLEY: I believe the answer — the quote was absolutely not. But it was a definitive no from him. Interestingly, he said, when I asked him why not, he said Bill Clinton in the White House. Basically vice president would be a ceremonial job.

It makes perfect sense. Most vice presidents are irrelevant, so why would a prominent Democrat want to have even less power than a normal VP?