Recently my Nokia 3310 headset's jack broke. As I couldn't find a pinout description anywhere on the web, I had to find it out on my own. To make life a little easier I decided to write up what I found:
The 3310's headset connects to the phone via something that looks like a regular 2.5mm stereo jack, only that it has four connectors. They were wired to the headset as follows:
A little Switch for answering incoming calls bridged the "Microphone +" and "Microphone -" leads.
Thanks to Bernard Ellis for proofreading the pinout.