We previouslyon a problem with iTunes Syncing after the iPhone OS 2.2.1 update. Now, a fix from Apple has materialized, and can be executed as follows:
Select "Deauthorize Computer" from the Store menu
Enter your iTunes Store Account ID and Password, then clicking on the Deauthorize button.
Select "Authorize Computer" from the Store menu
Enter your iTunes Store Account ID and Password, then click on the Authorize button.
Once this is completed you should be able to sync content again with your iPhone or iPod Touch. However, for some users, syncing content did not work again until after their next purchase from the iTunes Store.