My classic 80gig just did the same with Itunes 9.02 on my Macbook. Tried to restore, it wiped the music, wouldn't re-sync. After many hours messing, including restoring on 2 friends computers, reinstalled Itunes 8 (from here
http://support.apple.com/downloads/iTunes8_0_2_forMac ). Saved my current libraries to be safe, then simply dragged and dropped music folder into itunes. Then resynched my ipod. Took a while, but playing happily now. You loose your playlists etc this way, but the advantage is don't need to use another computer. For what it is worth, my ipod wouldn't resync immediately, I left it alone over night and then it was fine. Not sure why this would be, and am considering taking out the automatic sync function in order to charge it.
It is a hassle though, it would be helpful Apple to provide some explanation - appreciate not always easy to solve bugs, but information along the way would help retain customer confidence and loyalty surely. I am considering an alternative to Itunes (and the ipod) having read all this grief!
Anyhow, best of luck if you are having problems - this worked for me anyways
Cheers