The dreaded "itunes was unable to load provider data from sync services" error

iTunes users (as at version 10.3.1.55) on a network behind an internet proxy requiring a username and password to connect to the internet might get this error when they attempt to synchronise in iTunes. 
The error occurs because iTunes tries to access the internet to load data - and it does not correctly pass your login details to the proxy.  The good news is that there is a very simple work-around.

In iTunes, select the "iTunes Store" option in the left panel.  You will be asked for your proxy username and password.  Enter your details, and select the "Remember password" box.  Click the OK button.  This saves your details in iTunes for the duration of your current iTunes session.

Re-run your sync, and all should work correctly.  The only problem is that you will need to do this each time you restart iTunes.