When trying to upgrade to the latest iOS version (4.3.3) today I hit a rather annoying problem - itunes couldn't backup my iPhone and consequently wouldn't go ahead and run the upgrade.
I got a warning when connecting the iPhone via USB saying "iTunes was unable to load data class information from Sync Services. Reconnect or try again". But after dismissing the notification everything seemed to be working fine including syncing content to and from iTunes. However when trying to kick off the iOS upgrade it failed and advised to reconnect the iPhone. I did the reconnect, but the same happened. I rebooted both iPhone and MacBook but still the same problem.