Apparently there’s some incompatibilities with QuickTime 7.4 and Adobe After Effects 7 and CS3 (8). [LINK] Check out the comments for more information.
Update: It looks like the problem is also with Adobe Premier. It springs from the DRM associated with iTunes rentals. Apple and Adobe are working on it. [LINK]
Update: The problem appears to be fixed now, with the release of QuickTime 7.4.1. [LINK]