Interesting! The language on the MBP specs says "Combined headphone/line in", whereas the MBA specs say "Headphone port". But you are correct: my headphones register as an audio input on the Macbook Air. I wonder if it's a very specific exception, or just an intentionially undisclosed feature (why?).
I would read "combined headphone/line in" as meaning that it would do full stereo line-level audio if you provided it. The MBA's port only works with four-pin cell phone-style headsets, as far as I know, which do mono over an extra pin while still providing stereo output.