EDIT:
McScruffy said:
Contrary to popular media belief, the United States is not (entirely) filled with overweight, ignorant rednecks who can't formulate a single thought beyond "I wonder if a Big Mac tastes good WITHOUT the special sauce?". While we, as a nation, have slipped as a leading scientific mind for the world, there is still a lot of brain power on this side of the pond (eat MIT's 1000 GHz processor!).
Although the OP worded it poorly, I don't think he was trying to say that it's unlikely that the smartest person in the world would be American, but rather that it was just as likely they would be from another country (adjusted for population, of course). On the other hand, because America is currently a cultural and economic superpower, a huge proportion of highly talented and intelligent people emigrate to America if they aren't born there, in order to go to MIT, for example. So a lot of that American ingenuity is actually powered by skills drawn from the rest of the world. Take a look at the scientists associated with the Manhattan Project [http://en.wikipedia.org/wiki/Category:Manhattan_Project_people], for example, who created the first working atomic bomb: plenty of corn-fed Americans, of course, but also Hungarians, Germans, Italians, Russians, etc etc., many of whom moved to America in order to work on it.
In short: it's not so much the people, it's the cultural momentum.