As expected, an iTunes update is followed (or preceded, depending on how you download things) by a QuickTime update. Today, Apple has released QuickTime 7.5.5, citing "...changes that increase reliability, improve application compatibility and enhance security."
How typically Apple. Soon these release notes will simply say, "Stuff." Update: If you haven't installed QT 7.5.5 prior to installing iTunes 8, you'll get a warning from iTunes that movies won't play, etc. -- so you might want to do both at the same time.
The download is about 65MB. You'll find info on recent Apple updates here.
Thanks to everyone who sent this in!