Jamie Kirchick has a good piece in the NY Daily News explaining that the "birther" controversy is in fact manufactured by the left. He sums it up nicely:
It is not Obama's right-wing opponents, however, who are devoting the most attention to this obscure, Internet-driven "movement," if one can even use that label to describe such a paranoid groupuscule. Rather, it's liberals, bent on portraying their conservative opponents as extremists - and changing the subject to help a President under increasing scrutiny for the substance of his policies - who are driving this story.
Relatedly, Marc Ambinder of The Atlantic has written a useful explanation of the White House's strategy to paint the opposition as composed of birthers, people who believe the president is a secret islamist, and other extremists. Doing so is their best hope for getting their big projects -- such as health care -- passed despite the public's growing wariness.