Just had this error today with an iPhone 4 and the apple support document is not very useful.
From Googling, it seemed that many people used to blame this on corrupt photos in iPhoto – but tracking those down can be painful.
In my case I think it was a partial sync that happened when I drove away from the house in my car. I plugged in my iPhone in the car while in the garage, and it connected to my WiFi and started to sync – I was late so I drove away as it was trying to sync and then gave up. I think this made the iPhone very happy.
I was prepared to go down the route of doing a complete restore but I decided to try the semi-obvious first.
I rebooted the computer running iTunes, I shut down all the running apps on the iPhone and then rebooted the iPhone – and then it did a backup and then did a sync.
All is well. Hope this helps someone.