For some reason, all of my QuickTime movies have lost the generic Q logo icons. They now have the plain white icons when I view them in the finder.
Not something with which I've had prior experience, but if I were allowed to make some basic assumptions, "IMHO" I might conclude that either you have somehow lost the resource pointer or inadvertanly associated this file class with an iincorrect application.
In the first case, I would assume the problem can be easily fixed by simply opening the "Applications > Utilities > Disk Utility" application, sellecting the effected drive, and reparing the disk permissions. (This should be done periodically in any event -- especialy following the installation of new applications/upgrades or whenever you feel files are not operating optimally.
On the other hand, if you attempted to open your QT files in an alternative media (or inappropriate application) it is possible that you may have inadvertently reset the "Open with..." option for all of these files. In this case, you could simply hichlight the finder icon and open its "Get Info" window, reset the "Open with" option to the QT Player, and press the "Change All..." option to apply this selection to all files of this type..