As far as how the term "natural born citizen" would have been understood in 1787, it seems that a key requirement was that the parents (or at least the father) of the child were citizens at the time of the birth. So Obama does not qualify even if he was born in Hawaii--but neither does Rubio even though he was born in the US.
My theory is that the birth certificate that has been released is a fake not because the real one would show that Obama was born in Kenya but because of other information which would be damaging to him.