Well here is the catch, if you want a thermistor based circuit the simple resistance circuit wont work.
The circuit basically operates on the capacitor C1 charging time. The change resistance in R1 varies the charge time and hence the magnitude of equivalent AC voltage. However, considering the application you want, the charge time must be temperature dependent.
And since thermistor resistance variations are non-linear, you can't simply replace the R1 with an appropriate thermistor value(i.e., temperature increases->resistance decreases->charge time decreases->voltage increases->fan speed increases). But then again maybe you could try for a linear resistance variation thermistor for your required range of temperatures.
I can't seem to think of any simple analog implementation to this circuit. Any implementations with PIC will be much more painful as you will need to bother about synchronization of gate pulses with the source voltage. Any others want to pitch in ideas?