The fact that they will never release a thing of substance tells me this is a great issue to pound away on. Much like John Kerry and the Swift Boaters, what is he going to say? That he wasn't born here? That his BC has something on it which he wants to keep concealed? The answer is no to both.
Instead, all he can do is to attempt to discredit those asking the questions, just like Kerry attempted to do. Problem is (for Team Obama), the issue has become to widely known. The more he dodges the simple resolution to the question, the less credible he appears, even to some of his supporters.
Every time Obama opens his mouth, more and more Americans can see who he really is, and as a wise man once said: when your enemy is busy destroying himself, the smartest thing you can do is to let him.