iPod playback

Hey guys I just bought a 2009 Chevy Cobalt which has the usb port so you can connect a mp3 player and play all songs through the radio. I have a iPod classic and whenever I try to connect via the usb port it gives me the following error "iPod not authenticated. Check iPod version". In the cars owners manual it gives no suggestions other than ensure the iPod has the more current firmware installed which it does. The same thing happens to my new iPhone 3G. It plays through the auxillary port but I would rather use the usb port and take advantage of the controls that are built in through the radio for the iPod. Anybody have any ideas??