Obama has said that his wife won't let him run again. Take that for what you will.
Plus, I think in 2012, Hillary will stage a fierce comeback no matter who is in the White House when the campaign unofficially begins on Swearing-in Day in January 2011.
Obama certainly has time to run again, (he'll only be 51 in 2012), but he, and Hillary, will run the risk of being a has-been, the same thing that killed the Edwards campaign in early 2008 and probably part of the reason Al Gore isn't running.
This will sound very home-state biased, but I seriously think we will have a new player on the national scene in 2012, or definitely in 2016, and that is then-Former Governor Brad Henry of Oklahoma. Henry will position himself by running for the Senate in 2010 and use that as a spring board into either 2012 or 2016, or an outside chance of 2020. Remember, Henry will only be 47 by January 2011 (he was elected Governor at age 39), and that would put him 48 or 49 by Election Day 2012, and you can do the math for 2016 and 2020 and even 2024.