Only a day after releasing iTunes 10.3 that supports new cloud-based syncing between devices for purchased items from the iTunes store, Apple today quickly released a small update. Apparently, after the initial update a number of iPhone users were experiencing a problem with an "unknown error (-50)" showing up when trying to sync their devices. Various attempts at troubleshooting including resetting the iPhone and rebooting the Mac did not help, and it was apparent the problem was a bug with iTunes 10.3.
The latest update should address this issue, so if you were having this problem, you can use Software Update to check for and install the latest iTunes update. As with version 10.3, the iTunes 10.3.1 update should be around 75MB in size and can also be downloaded from the same support Web pages as the previous 10.3 versions:
Though this update should be a fairly simple and straightforward install, as always, be sure to back up your system and iTunes library before installing. If you use Time Machine doing this is as simple as selecting "Back Up Now" from the Time Machine menu and waiting for the menu icon to stop spinning.