Obama Wasn't a "Natural Born Citizen"???

I can understand the argument that Obama was born in Kenya-- it makes some sense, though it's hard to know for sure if that is the case. What I really don't understand is the argument that some people make, that Obama wasn't a "natural born citizen" of the US-- even if he was born in the US-- because his father was a British citizen. Does anyone understand that argument who can explain it to me? It seems the constitution does not clarify what "natural born citizen" means, but isn't the common law standard that if you were born on US territory, that you are a "natural born citizen"? Isn't that the whole point of "Anchor babies"?