QuickTime 7.4.5 update
In addition to the iTunes update, Apple also released an update to QuickTime tonight. Version 7.4.5 promises "fixes that enhance reliability, improve compatibility with third-party applications, and address security issues."
You can download this update by opening Software Update (Apple menu > Software Update).
Please note that after installing the iTunes update, you will be asked to install the QuickTime update for best video playback in iTunes.
Thanks to everyone who sen this in!