I agree with euromodder, the ECU or a mechanical device will raise the idle when the car is cold. I have also "fixed" this in my car.
for anyone interested i have written up in the wiki for how to reduce the cold start idle
Cold Start idle reduction - EcoModder
and the warm engine (standard) idle
Car idle reduction - EcoModder
for me both mods caused noticeable reductions in fuel usage but my engine seemed to be designed to rev fairly high when cold.
I have also never stalled with these mods (except when i stupidly put the car into drive from neutral when i have forgotten to turn off the torque converter lock up)