Obama was born a Brit.
Fallacy. The children born here of non-citizen parents have always been US citizens. It doesn't matter whether another country also extends citizenship to them or not.
He had an American mother. But even if both of his parents had been un-naturalized British immigrants, and even if the laws of England therefore made him a British citizen by birth, he would still be a natural born US citizen.
You might not like that, but that's the way it is.
As far as "dual citizenship" is concerned, that doesn't matter either. 3 of our first 4 Presidents were dual citizens of France, while serving as President. Nobody cared.
Just as nobody cared when John Charles Fremont, the first Republican candidate for President, ran for the nation's highest office while proudly touting that he was the US-born son of a French citizen who never naturalized and never intended to.
Demonstrably false.
Secretary of State Frederick Frelinghuysen determined Ludwig Hausding, though born in the U.S., was not born a U.S. citizen because he was subject to a foreign power at birth having been born to a Saxon subject alien father.
Similarly, Secretary of State Thomas Bayard determined Richard Greisser, though born in Ohio, was not born a U.S. citizen because Greisser's father, too, was an alien, a German subject at the time of Greisser's birth. Bayard specifically stated that Greisser was at birth 'subject to a foreign power,' therefore not "subject to the jurisdiction of the United States" within the meaning of the Fourteenth Amendment.
http://books.google.com/books?id=wdgxAAAAIAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false