The Constitution doesn't mention geography, that's true. That could either be because the framers intended it to apply to everyone everywhere, or because they thought it was obvious it would only apply to Americans in America.

Luckily, the Constitution is not a disembodied bit of text. We have copious context to use to evaluate the significance of the lack of geographical distinctions in the Constitution. The English legal tradition, which the Constitution is a product of, is highly territorial. English law applied to Englishmen in England. Thus, it is reasonable to conclude that American law, including the Constitution, was intended to apply to Americans in America.

Scalia's dissent in Boumediene has a good discussion of the application of Constitutional rights to non-citizens: http://www.law.cornell.edu/supct/html/06-1195.ZD1.html. I'd skip to Section II to get past the political rant.