I've been experimenting with using a capacitor to smooth the PWM frequency of a dc motor and wondered what are the limits of value you can use ie Are there dangers of using too high a value? Thanks, Goatboy 
See this thread it is going in that direction:- http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1240335161/15#15
Although it is talking about audio the same thing applies to motors or anything else you drive with PWM.
The limit comes when the capacitor is so big that it starts interfering with the speed you can change the speed of the motor. Also you could get into the situation when you hit a resonant frequency and you could see the voltage across the motor go up to 600V even though you are only powering it from 12V. The clamping diode will stop a lot of that happening but it is an issue.