Just wanted to mention that the QT 6.4 upgrade contains a showstopper bug for those of us using Final Cut Pro 4. It seems that any timeline in FCP that mixes media codecs (like say, DV material and an animation codec graphic) will not play and the never-ending beachball will freeze the program requiring a force quit. Many folks are reporting that they cannot even open their projects. The QT 6.4 readme suggests upgrading to FCP (mentioning DV improvements and better workflow). Obviously even casual testing by Apple would have revealed this bug prior to the release and they could have warned their hapless users. However even casual testing by Apple is apparently beyond the support they are willing to provide so many of us are now dead in the water.