I did it.
The idea was to use the AV (yellow-white-red) cable that came with my phone as stereo audio and a microphone. These would plug into the Nokia AD-54 remote control that came with my phone. I went to Fry's and bought all the components I'd need: a microphone, some couplers, and some wire.
The basic idea of the setup can be described as below:
This image (and all the others, 'cause I totally forgot it until the end) neglect the remote control that was really the main reason I made this setup. it's a little plastic box with some buttons on it, a wire to plug into the phone, and a headphone port:
It's the box on the left. This allows me to play/pause/stop/fast forward/reverse music and pick up/hang up calls, all pretty much without having to look away from the road.
My soldering is terrible so I managed to break a thousand joints during the process of installation. In fact, that was the single biggest holdup for this whole project. As you can see, I broke the connection for the right+ channel, so I followed the trace and soldered the wire directly to the surface-mount resistor at the end of its trace.
My soldering is so bad that I think some of it has already come undone: I noticed late yesterday a distinct lack of bass in my music that wasn't there in the morning. I'm planning to redo the joints with stranded wire some time soon, or just buy the $2 d-sub kit mentioned elsewhere in this thread, the crimp connectors from which I can connect to the changer port.
This was the basic idea for the mic. I wanted it mounted in the little blank panel on the dash next to the dimmer switch, where it could be pointed straight at me, since I had little faith in the $4 microphone I had bought. As you can see, I have subjected it to coupler hell and I was worried that the sound might not come through all right. I wanted to avoid soldering the mic cable because it would be subjected to a lot of jostling during the installation.
The cables run through here. I took this photo before deciding to run the switch through here as well.
The setup with the electrical work finished. It took a remarkably short time to get to this point - of course, I hadn't yet broken every solder joint twice, so I was excited about how quickly the mod was progressing.
I didn't have a drill bit exactly the right size as the microphone, but I did notice that my key was exactly as wide at the shank as the part of the microphone that I wanted to grab. So I drilled a 5/16" hole and reamed it out with my key.
It's a perfect press-fit. Tight enough that no amount of jiggling will move it, but just enough room to twist the mic if necessary. The switch I bought was listed on the manufacturer's website as needing a .25" hole, which was easy enough. In the process of drilling these holes, I managed to skin my finger and slightly break the panel I drilled them in. No biggie, though - it still sticks in place.
Dash bezel back in place. I wanted the mic and switch to be to the left of the dimmer control, but the couplers were stiff and wouldn't fit, so I had to move it inside.
The finished product. I forgot to take a picture of the remote, but you can see the cable poking out from the upper AC vents. This terminates in a 3.5mm plug (seen here) which is holding up the remote; the wire out from the remote then goes to a TRRS connector which plugs into the phone. This plug runs through a hole I drilled in the little tray below the radio and out of its door, so it can be tucked in when not in use. The phone sits in the ashtray. The only visible elements of the mod are the mic and switch on the panel and the remote chillin' next to the radio.
I wasn't too sure about the mic, but people say they can hear me clearly at highway speeds, so it seems to have been up to the job for less than $5, and that was the most expensive thing I bought. So I've got a good handsfree setup for under $15 in parts.
It also works for regular MP3 players and what-have-you with a 3.5mm jack, since in those applications the mic wire is just shorted to ground. It should work for regular phones using TRRS connectors as headset plugs too. Interesting note: because Apple is wack, they reversed the ground and microphone rings on the TRRS connector so that only iPhone-specific headsets work with iPhones and so that iPhone headsets work only with iPhones. Thus, my car's setup will work with any TRRS headset phone except the iPhone.