Keeping pace with other popular applications, on March 31 Apple released updates for both iTunes and QuickTime.
If you use iTunes frequently, you've probably already been prompted to update. If not, open iTunes, click Tools - Check for Updates and you'll soon be presented with a box offering to download and install updates to not only iTunes, but also Quicktime, Safari and MobileMe. Choose whatever you like, reboot and go on with your day.
iTunes is a big piece of sofware, and it takes some time to download, install and reboot. Get it started and go do something else for 15 minutes or so. It's probably time to stretch your legs and let your eyes refocus, anyway.
If you'd like to download the complete version from Apple, go here: http://www.apple.com/itunes/download/