This has also happened to Quicktime files recorded on my DR-100 Firestore drive from a JVC HD111 after iTunes 10.5 was released. I'm not entirely convinced it's iTunes as I did a complete system re-install on an erased hard drive from a Snow Leopard disc. On the initial install the files played played fine. I did a system update and prevented iTunes and Quicktime 7.6 from updating but the files no longer play. Booting from an older system drive enables the same files to play.
It also happened about 2 months ago after I let Quicktime 7.6.6 upgrade, the only solution was a system re-install.
At this stage Apple are blaming the cameras etc for this problem. Convergent Design have been in touch with Apple about this, so far no solution.
I get the feeling that it's not a priority at Apple to keep their Pro customers businesses running anymore, there's far more money in iPads and iPhones and associated apps. Buy a new iPhone 4s and you'll need iTunes 10.5 too I believe.