Right now, chances are you are that you’re probably downloading iOS 4.2. Chances are also that you may own more than one iOS device, such as an iPad and an iPhone.
Know that you can only download one update at a time. For instance if you choose “download only” rather than “download and install” iTunes will begin the iOS 4.2 software download. However, you will not be able to have it download the iPad software while you are downloading the iPhone software. Bummer.