I have a 3GS, I have 4.0 on my phone. I sync it to ITunes and I can add new apps and songs but as soon as it backs up it fails and gives an error (see pic). I also get this every time I try to upgrade to 4.0.1. Help. I have rebooted my PC, I have rebooted my phone. I just successfully upgraded my wifes phone with no issues.
I tried to update my sons itouch and it turned into a brick.
I let getting the similar error, but said couldn't back up due to error (-20). What I did was went into preferences and deleted my supposed corrupt back up file for my phone, then voil, it sync'd and backup'd for me. Not sure if this would help you. Note, prior to doing this, I restored my phone, so I had lost all my prior settings and data.