Some users are having problems with the iOS 4.2 download. The error message above is popping up for some, and users are having to deal with the frustration of having to restart the download.
It's a pain, but the best you can do right now is restart iTunes and begin the download again. It's probably just a result of thousands of iPhone and iPad owners downloading the heck out of the new firmware update.
We will keep tabs to see if it doesn't become a widespread issue. The most likely scenario is that Apple's servers are taking a hard hit right now as Uncle Steve does the happy dance knowing just what he will be talking about at Apple's next press event.