Check out this hysterically funny post by Jack Balkin! A tiny taste:
Suppose we say that the United States begins in 1775, so that Washington was a resident of the United States for fourteen years before he became President. Nevertheless, that doesn’t mean he was a citizen of the United States "at the time of the adoption of this Constitution." If you think the Constitution is a compact of states, which was legally adopted on June 21, 1789 when New Hampshire ratified, then "the time of the adoption of this Constitution" was June 21, 1789. But George Washington’s home state of Virginia didn’t ratify until June 25th. This means that at "the time of adoption of this Constitution," George Washington was not a citizen of the United States, he was only a citizen of Virginia, and therefore he was ineligible when he took the oath of office. And if he was not a citizen at the time of adoption, he could not be a "natural born Citizen" at the time of adoption either.
For my toughts (which pale compared to Balkin’s wit), scroll down or click here.