After a little searching, I found this page that details the electrical specs of the cable, and I set to work making my own.
Though I did find a computer CD-ROM audio connector to use to connect the cable to the camera, I wasn't as lucky as the guy who wrote that page when it came to a switch - the only switches I had available were of the simple "on/off" variety (SPST - single pole, single throw). So I put two of them together on a little piece of circuit board:
You can see the wiring below:
The red wire gets connected to pin 1 on the camera, and is connected to the white wire (pin 3) when the first switch is depressed, and connected to the black wire (pin 2) when the second switch is depressed. So, switch #1 is like holding the shutter button halfway down - it activates the meter and focuses the camera (in AF mode). Switch #2 is like pressing the shutter button all the way - it takes the picture.
Then, I needed a housing for the switches, so I made one out of wood:
After soldering the switches to the end of a telephone cable, I epoxied the circuit board in to the housing:
The wood lever hits switch #1 first, and then #2 if you squeeze a little harder.