You could try unchecking 'Display album artwork on your iPod' from the Music tab when you plug in your iPod. This will take off all the album artwork from your iPod. Then unplug your iPod and plug it in again and re-check 'Display album artwork on your iPod' and it will copy all of the artwork back on. This solved it for me, but I can not guarentee that it will work. Also, if you don't have USB 2.0 it will take a long time.