There are a few threads that discuss decoding an rc signal using the Arduino, such as this one: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1204020386/156#156
Note that the radio control signal isn't really PWM, (although it is often called that) it's a form of pulse position modulation (PPM).
A google search for Arduino and ppm should turn up some other links of interest