12 years ago.

You may want to start reading at http://en.wikipedia.org/wiki/Resistance_thermometer ...

PT-100s have a standardized but non-linear temperature/resistance relationship. If you can accurately measure the resistance, you need no calibration - just look up the temperature in the table (sample code at Wikipedia) or fit a polynomial. Note that to avoid self-heating, you should keep the current through the RTD low and/or turn it on only temporarily for each measurement, and you should measure in a 3- or 4-wire configuration to keep the influence of the sensor cable out. You will likely need to use an operational amplifier to scale the voltage to the ADC range.

For many applications I prefer PT-1000 which has the same characteristic, only 10x the resistance, and is easier to apply because of the larger resistance - in many cases you can ignore the lead resistance and just use a 2-wire Ohms measurement.