I just updated QT 7.4 on a spare machine for kicks and used AE CS3 as a render farm. It crashes 10 minutes into the render. Of course I didn't bother to research before upgrading. Apparently QT 7.4 has issues with Premiere Pro and AEFX CS3. Several forums have reported problems with the update.
Apple Discussion Forum on the same problem.
Sigh. beta tester anyone?