Take a look at the sourcecode for Linux Kernel 0.9 (First public version, IIRC)
Then take a look at the sourcecode for the latest kernel, 3.something something
I think y'all will find the mr. Xest is right about the codebase.
All that said... I seem to remember reading somewhere (and forgive me for the fuzziness of my memory here, please) that... uhm... some of the nuclear power stations in the USofA were still using IBM's OS/2 because it was far better designed, even security wise, than Windows (at the time). And besides, there was this "security by obscurity" thing about it as well.
So for the extra security which is built in to Unix systems (And Linux, *BSD, Darwin, and others that may be based on Unix) I would think it plausible that the USofA are utilising some form of Unix in their system. Perhaps heavily modified so as to not be recognisable as such (say, all commands and syntax being different, and the directory structure), but I could very easily imagine them doing something like that.
If for no other reason, that would probably make them quite safe from 90% of todays "hackers."
Just for the sake of asking a silly question (simply never heard the term before, figure that one out) doesn't "airgap" mean you need to physically access the computer somehow? As in, I wouldn't be able to sit "here" and access a computer "there", I have to be physically "there"?
(Sorry for the silly question, I'm still in work mode, where "airgap" mean the empty space between product and the shelf above.... aka space that does nothing... I called it a "whitespace" once and I got hauled arsefirst into the office and told "not to use non-company terminology" for that.... )