iGo decided to use a 4-pin connector for their tips. After some probing of the connector and of my tip with my multimeter, it was apparent that the first two pins are ground and power, connected directly to the barrel jack contacts. The last two pins are for adjusting the voltage and current limits of the power supply. The tip connects each limit pin to ground through a resistor whose resistance determines how high the limit is. My tip (I only had one to measure) had 13.9kΩ on pin 3 and 162kΩ on pin 4. By hooking up different values of resistors, I was able to watch the output change.
It appears that Pin 3 is the voltage limit, and Pin 4 is the current limit.
Pin 3 can have a resistance of anywhere from 2.5kΩ to infinity (open). 2.5kΩ sets the voltage to 24.5V and open is 15V. Any resistor in between can be chosen to get the desired voltage in that range. My 13.9kΩ tip tells the adapter to put out 16.6V for a Thinkpad laptop.
Pin 4 is a little harder to measure, since current limits require that you actually draw that much current. The tip had 162kΩ in it, which most likely corresponded to an amp or two. I actually found an article
about someone else configuring an iGo adapter and he has the resistances listed that he measured from 9 tips that he has. The only discrepancy is that he lists the current limit resistances as voltage limit resistances and visa versa.