http://www.usconstitution.net/consttop_citi.html
The ways to lose citizenship are detailed in 8 USC 1481:
* Becoming naturalized in another country * Swearing an oath of allegiance to another country * Serving in the armed forces of a nation at war with the U.S., or if you are an officer in that force * Working for the government of another nation if doing so requires that you become naturalized or that you swear an oath of allegiance * Formally renouncing citizenship at a U.S. consular office * Formally renouncing citizenship to the U.S. Attorney General * By being convicted of committing treason
Yep, all of which involves an adult formally and knowingly renouncing his citizenship.
If Obama was indeed born in Kenya, which I doubt, he would stll seem to be covered by this:
"Any one born outside the United States, if one parent is an alien and as long as the other parent is a citizen of the U.S. who lived in the U.S. for at least five years (with military and diplomatic service included in this time) ... Anyone falling into these categories is considered natural-born, and is eligible to run for President or Vice President. These provisions allow the children of military families to be considered natural-born, for example.