Here’s a fix that worked for me for an error that I got, while trying to sync an iPhone 3G to a macbook.
It was syncing 100% when out of the blue one day (on an important and a busy day) I get this error on iTunes saying:
iTunes cannot sync information with the «iPhone Name» because syncing has been disabled on this computer. Do you want to enable syncing?
When I tried to enable sync, the error message was
Syncing cannot be enabled on this computer