I remember reading alot about the older ones having these issues with different Ipod firmwares... Some even recommended going back to older firmware.. But being that the new Iphone 3G changed, I would suspect the same for the later gen Ipods...
This other adapter may be of some interest... I believe it just snaps inline with the other connector with no additional wires... I haven't read many reviews about it, other than people complaining about the price(which I kinda agree)... I was thinking about getting one myself...but I'm actually using my old 1st gen Iphone for Ipod music... I just leave it in my truck's console, and leave it powered on, as the Avic charges... But it would be worth a shot, although 40 bucks is quite a bit for such a simple little adapter...
http://cablejive.com/iphone_3g_charge_converter.html
That accessory error is stupid... I haven't found a single accessory that it doesn't say that for...my BMW is specifically made to work with the Iphone, and it still says that... cool thing is that the BMW charges both 1st gen Iphones and the Iphone 3G