Interesting idea HaTax, how does your fan currently behave with the rheostat, especially at lower speeds? Any issues with noise or motor heating? I assume you're using a rheostat meant for motor loads, not just a basic lighting one. If the knob on a rheostat just turns a pot, could you replace the output from the pot with a voltage signal from your Arduino board? This would eliminate the RC servo and the mechanical connection to the pot. I'm not familiar with Arduino, but I just looked at their website and they'd be fun to play around with if I had the time!