It depends what types of files you're talking about... Apple sells all of their content in MP4 files (even the audio-only music files are in MP4s), so that doesn't tell us much. iTunes doesn't sell "FairPlay" DRMed music files anymore, and you can "unprotect" (de-crapify) your existing files through the iTunes store (it costs either $0.30 per track, or I believe you can pay a flat fee for your whole iTunes library). When you do this, they also "upgrade" the quality on the files from roughly ~128kbps AAC to roughly ~256kbps AAC. I don't know exactly how to initiate this process, because I don't have any DRM-protected iTunes content, but I'm sure Google can tell you. If you don't want to pay, generally the easiest workaround that is guaranteed to work is to burn them onto disc (it can be a virtual disc if you're clever) and then re-rip them from the disc. This does result in a generational loss of quality, though. Whether that matters to you depends on your situation and needs (and how important the files themselves are to you).
All of their video content, and many audiobooks, is DRM protected and there is no official means by which to remove it. There might be hacks, I don't know, but if there are, it would violate the DMCA and I don't think discussion of them would be allowed in this forum. Apple literally cannot sell non DRM-protected video content from the "major" content providers. No one can. The studios and networks will not allow it. Anything you buy from the Amazon Unbox store is similarly protected, and most of the successful non-Apple services are actually streaming services, where you never really "own" a copy of the content in the first place (Hulu+, Netflix, Zune Marketplace, etc).
I don't know how MC handles playback of FairPlay protected content. I know in the past, MC could use QuickTime and iTunes to play back music so long as that file was authorized for play on that computer, and worked properly in iTunes. However, this was long-long ago and I don't think that system has been actively maintained in a long time, since Apple stopped selling DRM-laden music.
I don't think MC ever worked with DRM-infected video content from iTunes, but I could be wrong. I've actually never purchased a DRMed video (or song, actually) from iTunes, and I wouldn't, for this exact reason (rentals seem different to me, but I'm unlikely to rent anything because I have so much content already to keep me busy).
