Let me emphasize a portion of the FAM that I didn't before:
Chapter 7 of the Foreign Affairs Manual (7 FAM 1130, pg 8) says (and yes, I know it addresses children born abroad):
The Constitution does not define "natural born". The “Act to establish an Uniform Rule of Naturalization”, enacted March 26, 1790, (1 Stat. 103,104) provided that, “...the children of citizens of the United States, that may be born ... out of the limits of the United States, shall be considered as natural born citizens: Provided that the right of citizenship shall not descend to persons whose fathers have never been resident in the United States.”
This statute is no longer operative, however, and its formula is not included in modern nationality statutes. In any event, the fact that someone is a natural born citizen pursuant to a statute does not necessarily imply that he or she is such a citizen for Constitutional purposes."
As the referrence says above, the 1790 Act was repealed (in 1795). The 1795 Act was repealed in 1802, with parts amendments in 1855, 1934, 1941, 1952, 1954, etc... blah, blah... All STATUTES, right?
BUT, given that there is no current day statute that defines NBC in this Constitutional context, the Justices will look backwards to aid in interpretation. "What did the Framers mean with "Natural Born Citizen"?
The Justices will consider things like the Federalist Papers, Blackstone's Commentaries, and other documents of the time. They'll probably look at the differences between "citizen" and "subject," and how that relates to Art 2, Sect 1, Clause 5.
Those learned in the law in the framing era would have been familiar with Blackstone’s Commentaries, which James Madison described (in the Virginia ratifying convention) as “a book which is in every man’s hand.”
Blackstone wrote the following:
"Allegiance, both express and implied, is however distinguished by the law into two sorts or species, the one natural, the other local; the former being also perpetual, the latter temporary. Natural allegiance is such as is due from all men born within the king’s dominions immediately upon their birth. For, immediately upon their birth, they are under the king’s protection; at a time too, when (during their infancy) they are incapable of protecting themselves."
... and Barack Obama, Jr, was a British Citizen from his father at birth... it even says so on his website ...
Yeah, it'll be sweet... a defining time for our nation.