Thanks lefty, I wasn't sure whether the 100 in PT100 meant that the resistance was always 100 ohms nominal.
Mike, if you want to read it using the Arduino ADC then I suggest the attached schematic. The two resistors and the PT100 form a voltage divider that puts just over about 1V (the minimum recommended value) on the Aref pin. The PT100 gets about 0.5V across it, so it will dissipate 2.5mW. The 10uF capacitor ensures that the reference voltage is stable during the time it takes to perform a conversion. The 100 ohm resistor should have a low temperature coefficient, but the 820 ohm resistor is not critical. You will get a change in the value returned by analogRead of just under 2 per degC temperature change. If you want greater resolution than that, then I suggest you use an external ADC.
[EDIT: if your Arduino has a 3.3V pin, then a 430 ohm resistor to 3.3V would give a more stable reference than 820 ohms to +5V.]
