A disaster response team that I'm associated with just bought a 2011 Chevrolet Silverado HD 4WD Duramax Diesel Engine Pickup Truck. Here's what the dealer told us about radio codes:

"GM has a new procedure for radio codes. If power is lost and/or the radio is disconnected for any reason, the radio code will be lost and the radio will not work. A dealer must input the code because the codes change on a regular basis. Any dealer can re-install the code, but they have to get the code from GM on the day they do the work."

Hope this helps.