If he was born in Kenya, or anywhere else not in the US, he's not even a statutory citizen, unless later naturalized. His mother didn't meet the requirements of the statute (8 USC 1401 See the notes) as they existed at that time. Either way, non-citizen or naturalized, he'd not be eligible to the Office of President.
He's also admitted to dual citizenship at birth. Either a British subject or Kenyan citizen. Not natural born. His father was a Kenyan citizen and therefore a British subject and Stanley Ann Obama (nee Dunham) didn't meet the requirements to confer US citizenship on B. Hussein.
7 FAM 1131.6-3 Not Citizens by “Naturalization” (TL:CON-68; 04-01-1998) Section 201(g) NA and section 301(g) INA (formerly section 301(a)(7) INA) both specify that naturalization is "the conferring of nationality of a state upon a person after birth." Clearly, then, Americans who acquired their citizenship by birth abroad to U.S. citizens are not considered naturalized citizens under either act.
7 FAM 1131.2 Prerequisites for Transmitting U.S. Citizenship (TL:CON-68; 04-01-1998) Since 1790, there have been two prerequisites for transmitting U.S. citizenship to children born abroad: (1) At least one natural parent must have been a U.S. citizen when the child was born. The only exception is for a posthumous child. (2) The U.S. citizen parent(s) must have resided or been physically present in the United States for the time required by the law in effect when the child was born.
http://www.state.gov/documents/organization/86757.pdf