Close political watchers know that the right wing fringe that has captured the Republican Party is President Obama's best friend. When liberals, union members and non-authoritarians point out the harm Obama has done to progressivism all the corporate Democrats had to do was point out that, yes, Obama hasn't been that hot, but look at the crazies he's up against. That strategy, called Peak Wingnut Theory by Spot, holds that Democrats should do all they can to drive up the "legitimate" crazy element in the Republican Party, in effect ghettoizing Republicans within their own minority mindset. The crazies on the right enable Obama to drive more knives into the backs of his supporters, while creating space he can move into to his political right.
The status quo ante before Obama released his long-form birth certificate was a perfect balance for both dividing the Republicans and marginalizing them with the general electorate. Rational people knew that Obama was born in the U.S., and the president had the documents to prove it. But wingnuts like Donald Trump could find enough wiggle room (The president hasn't released his long form birth certificate! Why not?) to keep the rabid right riled up. This was forcing other Republican presidential aspirants to toe a thin line on the birther issue, not really embracing it, but not really repudiating it either.
Establishment Republicans have already tried to ostracize the birthers. Today the president did the Republicans' dirty work for them, saving them from having to savage candidates of their own party. With the release of his original birth certificate the birther issue has been marginalized even within Republican circles. Sure, there will always be parts of the crazy right who deny the president's legitimacy no matter what, but that faction has been reduced because of Obama's action today, opening the door to more moderate-looking candidates like Mitt Romney or Tim Pawlenty. To the extent that more serious Republican candidates not tainted by the loony birtherism are more appealing to conservative to moderate Democrats and Independents Obama has just ensured that he will face a more formidable candidate in 2012.