Like The Dish and Jonathan Bernstein, Nate Silver takes a closer look at Obama's approval numbers. Silver shows that "there has been almost no relationship between a President’s approval rating following the midterms and how he eventually fared at the next presidential election":
Franklin Roosevelt in 1943 and Dwight Eisenhower in 1955 had the highest approval ratings at this point in time and indeed won their elections fairly easily. On the other hand, Ronald Reagan was tied with Gerald Ford for having the lowest one 37 percent and he won in a blowout. Bill Clinton’s approval rating was 47 percent, and he won. George H.W. Bush’s was 58 percent, but he lost. His son George W. had a 63 percent approval rating and won, but only barely.
As you can see, there’s not much of a relationship there.