You don't have to be born in the US to be a natural born citizen. For instance, if you were born on a military installation, you are still a citizen. Otherwise McCain wouldn't be a citizen, (and therefore not allowed to be president) since he was born in Panama IIRC.
McCain was not ruled a natural born citizen because he was born on a US military base. It was because he was born with a Father and Mother who were US citizens.
http://en.wikipedia.org/wiki/Natural-born_citizen
Despite widespread popular belief, U.S. military installations abroad and U.S. diplomatic or consular facilities are not part of the United States within the meaning of the 14th Amendment. A child born on the premises of such a facility is not subject to the jurisdiction of the United States and does not acquire U.S. citizenship by reason of birth."[17]
John McCain (born 1936), who ran for the Republican party nomination in 2000 and was the Republican nominee in 2008, was born in Colón, Panama,[36] near (but not part of [37] [38] [39]) the Panama Canal Zone[40] of two U.S. parents, who were at the time serving at the Coco Solo Naval Air Station.[41] In March 2008 McCain was held eligible for Presidency in an opinion paper by former Solicitor General Ted Olson and Harvard Law Professor Laurence H. Tribe.[42] In April 2008 the U.S. Senate approved a non-binding resolution recognizing McCain's status as a natural born citizen.[43] In September 2008 U.S. District Judge William Alsup stated obiter in his ruling that it is "highly probable" that McCain is a natural born citizen, although he acknowledged the possibility that the applicable laws had been enacted after the fact and applied only retroactively.[44] These views have been criticized by Gabriel J. Chin, Professor of Law at the University of Arizona, who argues that McCain was at birth a citizen of Panama and was only retroactively declared a born citizen under 8 U.S.C. § 1403, because at the time of his birth and with regard to the Canal Zone the Supreme Court's Insular Cases overruled the Naturalization Act of 1795, which would otherwise have declared McCain a U.S. citizen immediately at birth.[45] In any case, the US Foreign Affairs Manual states that "it has never been determined definitively by a court whether a person who acquired U.S. citizenship by birth abroad to U.S. citizens is a natural born citizen […]".[46] In Rogers v. Bellei the Supreme Court only ruled that "children born abroad of Americans are not citizens within the citizenship clause of the 14th Amendment", and didn't elaborate on the natural born status.[47][48]