According to the U.S. Constitution, it states that a president needs to be a "natural born citizen" of the United States. "Natural Born" means that he was born in the United States and that both of his/her parents are American's.
Barack's precise location of birth is still being investigated. But according to the Constitution, he is not a "natural born citizen" due to the fact that his father was Kenyan born and held a British passport. Only his mother was American.
Therefore, Obama is ineligible to serve as president.