He did have to have a U.S. citizen dad to be an NBC at the time he was born.
http://www.freerepublic.com/focus/f-news/2302859/posts
You can only reach that conclusion by ignoring SCOTUS rulings like Wong Kim Ark and undoing modern understanding of birthright citizenship. But leave aside that boutique theory which has nothing to do with his birth cert.
Where was Obama born?