Linus Torvalds saith:
... the reason GPLv2 is so successful at fighting the root problem of using copyright to fight restrictive copyrights is that it makes "interesting material" available under a license that forbids further restricting it.
I would suggest that anybody who wants to fight DRM practices seriously look at the equivalent angle. If you create interesting content, you can forbid that _content_ to ever be encrypted or limited.
In other words, I personally think that the anti-DRM clause is much more sensible in the context of the Creative Commons licenses, than in software licenses. If you create valuable and useful content that other people want to be able to use (catchy tunes, funny animation, good icons), I would suggest you protect that _content_ by saying that it cannot be used in any content-protection schemes.
Afaik, all the Creative Commons licenses already require that you can't use technological measures to restrict the rigts you give with the CC licenses. The "Share Alike" license in particular requires all work based on it to also be shared alike, ie it has the "GPL feel" to it.
If enough interesting content is licensed that way, DRM eventually becomes marginalized. Yes, it takes decades, but that's really no different at all from how the GPL works. The GPL has taken decades, and it hasn't "marginalized" commercial proprietary software yet, but it's gotten to the point where fewer people at least _worry_ about it.
As long as you expect Disney to feed your brain and just sit there on your couch, Disney & co will always be able to control the content you see. DRM is the smallest part of it - the crap we see and hear every day (regardless of any protection) is a much bigger issue.
The GPL already requires source code (ie non-protected content). So the GPL already _does_ have an anti-DRM clause as far as the _software_ is concerned. If you want to fight DRM on non-software fronts, you need to create non-software content, and fight it _there_.
I realize that programmers are bad at content creation. So many programmers feel that they can't fight DRM that way. Tough. Spread the word instead. Don't try to fight DRM the wrong way.
I agree on the need to license "content" under less restrictive licenses. But I'm still not convinced that using copyright is the right way to go. In particular, I think that putting your works into the public domain may well be the best avenue to prevent DRM schemes from proliferating. Consider what would happen if someone tried to distribute the works of Shakespeare under DRM. Would that threaten the open nature of Shakespeare's works? No, it would only cause people to laugh at those who attempted to DRM Shakespeare in the first place. IMHO the best protection against those who would close off content is to make it part of the public domain, distribute it as widely as possible, and publicize its existence and its freedom.
Those who would restrict access to ideas hate the open air. Making your ideas free as the air gives those people nothing to lock up and tie down.