Apparently, Congress can rule Obama is a natural born citizen even if born in Kenya if they so choose.
Of course, this is Wikipedia so it may not be correct
http://en.wikipedia.org/wiki/Natural-born_citizen
Section 8 of Article I confers on Congress the power “to establish a uniform Rule of Naturalization...” This power has been construed to include defining the characteristics of a “natural born citizen”, as well as the conditions of “naturalization”.
The 1790 Congress, many of whose members had been members of the Constitutional Convention, provided in the Naturalization Act of 1790 that “And the children of citizens of the United States that may be born beyond the sea, or out of the limits of the United States, shall be considered as natural born citizens.” In addition George Washington was president of the Constitutional Convention and President of the United States when this bill became law. If Washington disagreed with this definition, he could have vetoed this bill. However, in 1795 the Congress passed the Naturalization Act of 1795 which removed the words “natural born” from this statement to state that such children born to citizens beyond the seas are citizens of the U.S., but are not legally to be considered “natural born citizens” of the U.S. This was done to clarify for those living at that time who was and who was not a “natural born citizen” per the framers intent at that time, since the 1790 Act had introduced confusion into that subject in regards to the use of those words in the Constitution. George Washington was also President in 1795, and thus he was aware of this change. And if he disagreed with the clarification and change in the wording in the new act in 1795, he would have vetoed the Naturalization Act of 1795.