OK; I've seen a lot of the bull that gets passed along as being valid by folks that should know better, so I dug out some info from my brother and then hit the American Consular Site at
Hamilton, Bermuda for verification.
A child born outside the United States or one of its outlying possessions to U.S. citizen parent(s) and your U.S. citizen parent(s) meets the * physical presence requirements.
* Physical Presence
This is the actual time when the parent was physically present in the United States, not simply as a resident. This means that any travel outside the United States, including vacation, should be excluded. Evidence or proof of physical presence/residence is required. This can be demonstrated by providing documents such as school transcripts, W2 (tax returns). The consular officer will determine whether or not you have met the requirements and demonstrated adequate physical presence.
Note: Any periods of time spent overseas with the United States Military/Government etc. may be computed as physical presence in the United States for transmission of citizenship purposes. Time spent as a dependent of a United States Military/Government employee may also be computed as physical presence. Military records may be requested.
To claim the child under US Laws, you not only need to provide parental citizenship, but also parental residency compliance. If Obama's Mother did not maintain her residency then there are other hoops for them to jump through that allows him to be a Naturalized Citizen, and unqualified for the Presidency.