Here's an interesting article about QuickTime X:
Apparently it still uses QuickTime 7 in the background quite a bit:
"For every one of the cases where QuickTime X is not up to the job, QuickTime 7 will fill in. Cutting, copying, and pasting portions of a video? QuickTime 7. Extracting individual tracks from a movie? QuickTime 7. Playing any movie not natively supported by an existing Apple handheld device? QuickTime 7.
Augmenting QuickTime's codec support using a plug-in of any kind? You guessed it: QuickTime 7."[/b]