I could answer this myself tomorrow morning by trying it out on my fiancée's new MBA but it's in the bedroom and she's asleep. Chances are I'll forget by tomorrow so I figured I'd ask here. My Unibody MacBook Pro (Rev A) can use the microphone in my iPhone headphones when they're plugged into the audio out port. The volume controls on the headphones also work. Is this also true for the audio out socket on the MBA? If yes then it should be pretty easy to make a mono audio input source by hacking an old set of iPhone headphones. I'll probably just get some kind of USB audio in adapter but this could always be an option.