DRM-enabled movies, i.e. the vast majority purchased on iTunes, appear to be blocked when it comes to taking screen grabs in Mountain Lion. Since this happens at a low level under control of the operating system, it's extremely unlikely that you'll find any kind of app or command shortcut for Mountain Lion that will take screen grabs of this kind of movie.
People who say this works or that works are most likely talking about movies that are not protected by DRM.
I have to say that I find this to be extremely annoying, since I often take screen grabs from movies for review or discussion purposes (which I understand falls well within fair use policy). This is a million miles from copying a movie. Needless to say, I don't take such screen grabs on my Mac on Mountain Lion, because I can't.
Luckily other ways remain. Whilst highly inconvenient, you can still take such screen grabs (today at least) on an iPad.
In my opinion, Apple should drop this stupid restriction, as it's not really doing anything to protect anyone's rights; it's just an inconvenience for film buffs.
