I've seen that discussed before (not in the case of McCain, but actually as a discussion in a government class in high school), and American citizens born on military bases off of American soil are the exception to the "natural born" term... and as I understand it, one of the reasonings I've hear is that US military bases are actually considered American soil while we hold that ground (same with embassies).
To be completely honest, classy or not, it was a no-brainer on Obama's part. Imagine the backlash he would recieve had he argued McCain's legitimacy as a candidate because of where he was born. His father was serving in the military!
It would not only be the mistake that Clinton needed to eek her way into the nomination, it would quite possibly be the mistake the Republicans need to maintain their hold on the White House.
In short, even considering touching that topic would be campaign suicide. Therefore, dumber than snot.