For starters, let's note that it takes some gall to make fun of Rush for this mistake just a scant week after every major MSM outlet broadcast completely fabricated quotes of his, originally created out of whole cloth and disseminated as part of a deliberate effort to portray him as a racist.
Second, let's consider the larger context of this issue. What's interesting about hoaxes like these is that they're often boomerangs. Sure, people can put an anti-Obama rumor like this out there in a supposed effort to showcase just how gullible and hateful the right is. The ultimate problem, however, is that not everyone who heard the original claims is going to hear the apology for the mistake. Even when they do, the rumor has nonetheless gotten "out there" and some people are going to believe it, no matter what. And in this case, that phenomenon will hurt the President.
As wrong and as malicious as the false rumors about Rush are, in the end, they aren't that big a problem for him. He isn't a politician who needs votes to survive. He has a huge megaphone to set the record straight, along with an enormous core group of listeners -- and as long as his insight remains sharp (as it is) and his ratings remain high, he'll be fine. Nor is this sort of thing a problem for politicians like George H.W. Bush, who had held numerous government posts and whose life was an open book. Or for a Ronald Reagan, who had been in public life forever when he became President. Or even for a George W. Bush, who came from a well-known, much-vetted family -- and who was open about his own shortcomings as a student and otherwise.
But it is a problem for someone like Barack Obama, who practices what Politico has aptly described as "the politics of personal perfection." Couple that with the fact that much of his life -- before law school and even afterwards, as a "community organizer" -- has remained largely unscrutinized by the MSM. And add to that the fact that he has refused to release large chunks of information about the relatively short career he has had (where are the grades from Columbia and Harvard? Where is the original manuscript for "Dreams from My Father"?), and you have the perfect recipe for a hoax. When people haven't been given the information they need to form an accurate impression of someone, they are more likely to use misinformation to fill in the blanks.
So members of the MSM can chortle with elite condescension about the short-sightedness of Ledeen, Limbaugh and all. But to the extent that the original, incorrect claims remain floating around in the ethos long after the facts have been established, the joke ultimately may be on their hero, Barack Obama, rather than those who were fooled by the lies.