It appears that the problem stems from Microsoft's HDAudio.sys driver:
People are frustrated by the fact that the microphone on their headset is properly detected in OS X, but not in Windows/Bootcamp. In these, a single 3.5 mm jack connector is used to combine a stereo pair OUT with a mono microphone IN. The problem has been around for quite some time now, i.e., since the so-called TRRS jacks were introduced in MacBooks.
