Consider that most of the available heat energy is stored in the mass of the tip not the heating element. Think of it as a reservoir for the heat you want to use and the element as a trickle of heat to replenish it. Given a few seconds the trickle of heat will replenish the store in the tip. The element itself is relatively isolated from the soldering point, both in terms of heat coupling and the time it takes for a change in tip temperature to reach it. The element in most soldering irons is a coil around or inside the barrel so there is likely to be a temperatue difference between a cooled down tip and the end of the coil nearest the handle. As the temperature of the tip cools quickly, the end of the heater will cool slowly (dropping resistance) and the temperature drop will tend to incease the current in the other end of the heater increasing it's resistance. It means the overall resistance is a poor indicator of instantaneous tip temperature.
Curie point control is very effective but mechanicaly difficult to make. Thermistors and other resistance based methods will always has a slow response and be some way from the soldering point. I've never tried it but perhaps a method of detecting IR would work, looking at the radiated temperature, maybe looking down the center of the shaft with the detector in the handle. A modified PIR sensor perhaps.
Brian.