Actually, both is parents had to be U.S. citizens. Just the fact that his father was not a citizen makes him ineligible. That is besides the fact that he probably held dual citizenship, not to mention that he was adopted by his step father and no record (so far) that he ever legally changed his name back to Obama. He is ineligible on soooo many levels.
Does this mean that the children of illegal aliens born in the US are not eligible to be POTUS?
I do think that Obama is in big trouble, on this issue, if he was NOT born on US soil, as his mother was not old enough to automatically pass along citizenship.
However, there is NO requirement that both parents needed to be US citizens.