Barack Obama's Birth Certificate - Proof of Citizenship?

This is Barack Obama's birth certificate. It clearly states that he was born in Hawaii to a mother that is indeed an American citizen. How come this is not considered proof of his citizenship?

Two lawsuits has been filed against the presidential candidate claiming he is not a valid citizen of the United States Of America and thus not qualified to run for office according to the U.S. constitution. But surely this document proves that he is indeed a natural born citizen of the United States and should therefore be eligible to run for president? What's really the problem here?

Read more about the Marquis and Berg lawsuits concerning Barack Obama's citizenship in this article