I mean, it's pretty simple.
The US Constitution mandates that only a "natural born citizen" can be President; and this has been held to mean someone born in the US or one of it's territories. However, the Constitution does not provide a means of proving this.
So, we go down to the next level, which would be the US Code. In the US Code, the authority to determine US Citizenship is delegated to the Department of State. The Department of State has published, within the Code of Federal Regulations, a list of what pieces of information need to be on a document from a state registrar to prove birth within that state.
The document supplied by Obama meets all of these requirements. Therefore, according to current US law, Obama has met the requirement for proving he is a "natural born citizen." The document can have any title in the world on it - it could be titled "Ibitty Bibity Boo" instead of "Birth Certificate" and would be sufficient to prove his birth in Hawaii, since it has all the correct information and is notarized.
The only step at this point would be to somehow show that the notary committed fraud - which would be a long shot, and pretty hard to prove. Especially since there at least *three* notarized statements from different people to the effect that the first document is accurate. So, now you have to prove collusion between all four notaries.