Some of the older iPods (3rd-gen, with the four buttons above the scrollwheel) had the ability to use one of the headphones (the left one, I believe) as a microphone. You could go into the firmware of the iPod and record short, ~10 second clips of audio. It wasn't great, since you had to speak directly into the headphone (it wouldn't record ambient noise, like a lecture hall or anything).
Belkin offers a microphone adaptor that may or may not work with your particular model of iPod:
http://catalog.belkin.com/IWCatSecti...ection_Id=2062