I am having the exact same issue.
I have tried mov files from mutiple sources such as my iphone, itunes and videos converted from my HD Camcorder and none of them will play in quicktime x, i can force them to play in quicktime 7, but for some reason not in x.
I am also unable tp play any of the videos on the apple homepage, or for that matter play or download movie trailer from the apple trailers page.
If I use Firefox, i can play the embedded videos from apple (both homepage and trailers) however i cannot choose the download option for the movie trailers.
I also cannot ge the mov files to play in itunes either and this includes videos download from itunes on windows 7 pc rather than mac.