If Biden runs again in '16 and manages to get elected, I think he would decide not to run for reelection in 2020 and let whoever he'd pick as VP to have the honor.
More than that, I think he'd state flat out in 2016 he plans to seek only one term. (And I think there's a better than even chance he'd pick a woman for a running mate, maybe Hillary.)
But the way the Obama Presidency is going, we're likely to see a strong "outsider" challenge. The Democrats with outsider identities in the last 3 elections were Bill Bradley, Howard Dean and Barack Obama. Each did better than the last. If Obama is deemed a disappointment by the base, you'll see probably a few reformers through their hat in the ring: Schweitzer, Feingold, Sherrod Brown for example...
I'm not sure if he would make a one-term pledge just because that would make him a lame duck President from the get go. But who knows? It sounds like McCain was pretty close to doing it. I do think its pretty likely that the Dem candidate for VP will be a woman, if the nominee isn't one already. Gillibrand comes to mind.
