More attention to the question of whether Obama was born in the United States probably helps Democrats and hurts Republicans. This is an issue where you've got controversy within the Republican Party, but everybody else knows what's going on, and elite media opinion is firmly on the right side. So Republican politicians have a choice: either reject the conspiracy theory, alienating some of their primary voters, or treat it as credible and look crazy to everyone else.
The effect of Obama's request for a birth certificate is to raise the salience of the issue nice and early in the Republican primary. I don't think it'll put any dent in the number of Birthers. The people who remain unconvinced right now are deep enough in a conspiracy theory (as Nick notes below in linking to my earlier post -- one they may accept for reasons that have little to do with evidence) that there's no way evidence is going to pull them out.