I wouldn't put much stock in that talk. Gallup's figures are similar -- it had Bush's job approval rating at 34 percent when he left office and 47 percent now -- but it also notes that almost every ex-president starts to poll better in retrospect. Check out the other leaders' numbers here and see for yourself.
Here is Gallup's explanation for the pattern:
Americans tend to be more charitable in their evaluations of past presidents than they are when the presidents are in office. Former presidents likely transcend politics when they leave office, moving into a more nonpolitical role compared with the highly political environment in which presidents operate. And Americans' retrospective views of presidents may focus more on their accomplishments as president rather than the day-to-day political decisions or the state of the nation that are big influences on their approval ratings while in office....
Evaluations of presidents may also be influenced by their works after leaving office, which tend to be charitable in nature -- such as the fundraising for Hurricane Katrina led by Bill Clinton and George H.W. Bush -- or diplomatic, such as Carter's involvement in negotiations to secure the release of political prisoners.
That's a sensible set of theories (though I suspect that Carter's strong numbers are influenced more by his charitable work, which most people admire, than his diplomacy, which is controversial). But there's something else here that has to be acknowledged, a byproduct of the apocalyptic strain in politics. When a president is in office, there's always the possibility that he'll be the one who finally blows everything up -- the man who launches a nuclear armageddon, oversees the complete collapse of the economy, or otherwise ushers in the endtimes. Once he's left the White House and we move on to worrying about some other executive, the old guy starts to seem comparatively harmless. The Bush years saw the biggest terrorist attack in U.S. history, a storm that had some people worried that New Orleans would disappear altogether, two wars, and the worst economic crisis in years. At the time that may have seemed like the end of the world, but now it's something we survived. When you look at that uptick in Bush's numbers today, you're seeing a sigh of relief.