Obama's mother was born in the US. The only reason he would be in trouble if he was born in Kenya is her age. Congress could always pass a law redefining "natural born citizen" to include anyone who had at least one parent who was an American citizen, regardless of age, and that would be that.
For that matter, they could redefine "natural born citizen" so it included being born in the US or its possessions or in Kenya, and who's to stop them?
Again, that "nasty" thing called the Constitution. Ex post facto laws are explicitly prohibited!