As some have pointed out, President Obama's recent polling surge has only a modestly suggestive impact on his reelection odds, but but one clear effect is to dissuade potentially strong challengers. Such as John Thune:

South Dakota Sen. John Thune just landed two plum posts in the Senate — seats on the influential Finance and Budget committees — that crystallize the decision he now faces: Stay in the Senate to climb the leadership ladder, or jump into the 2012 presidential contest.
The betting among Republicans is that Thune remains in the Senate, and the senator himself acknowledges he might stay put.
“It’s a different scenario maybe than some of the other candidates who don’t have a job,” he said with a laugh Tuesday in an interview in his Senate office. “This is a great place. This is a great job. We all complain about it. It’s frustrating at times ... but it is a place where ultimately you can be engaged in the big debates about the issues."

As Thune notes, many of the other contenders do not have jobs, so they're pretty much all-in on 2012. But Thune can pick when he wants to run for president, and running against a fairly popular incumbent president may not be the best time to pick.