How to read RPM from 3-wire fan - PIC18F4550
Here's a specification for 4-wire (PWM controlled) fans from formfactors.org "brought to you by Intel": http://www.formfactors.org/developer%5Cspecs%5C4_Wire_PWM_Spec.pdf


  Tachometer Output Signal Fan shall provide tachometer output signal
  with the following characteristics: 
  
  
  Two pulses per revolution
  Open-collector or open-drain type output
  Motherboard will have a pull up to 12V, maximum 12.6V
  


Note that the pull up is supposed to be on the motherboard, i.e. not on the fan.