I've been watching the PWM frequency related threads lately. You may find these useful in clearing some things up. See specifically the posting by macegr in the first link below:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1235060559/8#8
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234764073