Biden does have some advantages. As VP, he can raise money, campaign for other Democrats and collect political chits. Hillary cannot due to her position as Secretary of State. But Bill can fill in for Hillary on the political circuit as she attends to the business of governance, which undercuts that advantage. Biden also has closer proximity to President Obama, which is an advantage Hillary cannot replicate.
If Clinton is running for president in 2016, she resigns as SoS by January 2015 at the latest. When it gets to the point at which one has to start raising $, she will already have left office.
But unlike for the 2008 election, her age will be an issue of course. Whether Biden being a prospect would heighten that issue for Hillary (two older front runners) or distract it from her is unclear. I assume the polls will begin in November 2012 actually and that Hillary will lead early ones by a comfortable margin with Biden generally second. The only other people to register might be speakers who did well at the 2012 convention (like Obama used in 2004 to launch himself to stardom). Schweitzer's a good bet here. Also, maybe surrogates who gave strong performances in the months up to the election.
If you're talking about 2016 polls taken in 2013, no one else will register besides Biden and Clinton. Having a good speech at the convention and being a campaign surrogate doesn't make you nationally famous. (Obama, for example, was *not* that well known nationally until early 2007. The 2004 convention speech only made him famous among political junkies.) I don't see how any new faces on the Democratic side become famous in the next four years, aside from Cabinet secretaries. So I see the earliest 2016 Democratic primary polls being dominated by Biden and Clinton, with everyone else being in low-mid single digits, like Haley Barbour and Tim Pawlenty are now re: 2012.
EDIT: The only possible exception would be if some kind of crisis catapults some other politician to national fame, a la Rudy Giuliani on Sept. 11, 2001.
It IS an interesting question, what would Obama do if both Biden and Clinton jumped into the 2016 Dem primary? Probably stay neutral is my guess, although one never knows what is going on behind the scenes.
If Obama wins reelection in 2012, he will certainly remain neutral in the 2016 Democratic primary, regardless of who runs....just like Reagan in 1988, Clinton in 2000, and Bush in 2008.