For anyone wanting to build a temperature sensor version then this set of 4 videos is very interesting. The guy explains how to use a LM35 as a temperature sensor and PIC12F675 to control the PWM fan.
YouTube - PWM Fan Controller - Intro and LM35 Temperature Sensor
I think the microcontroler needs programming and this is not explained so i hope killerbug can come up with a guide for a PS3 specific version