The latest iTunes (version 10.3) just shipped a couple days ago but it appears some users were encountering the “unknown error (-50)” when trying to sync their device. Even though there seemed to be some workarounds around that, it was clear that iTunes itself was the cause of the problem.
Today, Apple shipped iTunes 10.3.1 which apparently fixes this issue. If you were having syncing issues with iTunes 10.3, make sure to hit the “Check for Updates” option in iTunes to download the latest version…
This version also allows you to use Automatic Downloads with apps and books or download previously purchased apps and books from your computer or iOS device.
Let us know if you’re still encountering issues after updating iTunes.