At least in our lifetime, until Obama, there's never been a president whose U.S. nationality has been in possible dispute. But presidential candidates are required to show proof of their native-born citizenship, to even be eligible to run.
(Oddly, only his rival John McCain comes close to having his native-born eligibility in question, since he was born on a militasry base outside the U.S., and I recall an element of the left questioning McCain's eligibility during the 2008 campaign at one point)
You're right, though, that Obama plays to his base and makes the birthers look like wackos by simply not providing the proof that I'm fairly certain he could provide any time. While I think the request is legitimate, it still is providing nothing but posturing and political theatre.
