When I play some of my "old" QT movies from 9.1, it starts playing them in QT 5.0 thru Classic. Why is that ? It should be using the qt in OS X, right ? The same thing happens with some MP3´s thru Audion. erh...
I've noticed some odd behavior like this with old docs and stuff. The best fix I've found for it is:
Rather than double-clicking the icon to open something, either open the application you want to use first, then use the OPEN command and open the old file. Then save it. Or, drag the icon you want to open to the icon of the app you want to use to open it (either in a finder window/desktop, or in the Dock) and drop it. The system will force it open with the new app, but be sure to save it. Once I did that, it always opens within OS X, if it can.
If you have carbon and non-carbon version of apps (or cocoa and non-carbon), you may want to delete the non-carbon versions off your hard drive. In this case, delete the old quicktime player that you had, and let X and 9 use the new one. The same goes for SimpleText.
If you don't want to delete things right away, just archive those old programs with DropStuff or tar.
After deleting or archiving the old programs, X will use the next available program which hopefully is X-savvy. Try using Sherlock to locate all copies of offending programs (particularly SimpleText).
You can also force an application to open a file. Hold down the Option key as you drag a file to an application's icon (either in a window or in the Dock/Shelf) -- this will force the app to attempt to open it.
this is also good for files that seem to lose their creator info...