A few Linux distributions emphasize license freedom as a goal, but Tom "Spot" Callaway says that there's no such thing as a free Linux distro.
Callaway, Fedora's engineering manager, writes that "you'd need to do an incredibly thorough audit of every file in the system to be sure that every single file is under a known license," and the evidence says that most distros haven't actually done it.
When we find something non-free, it almost always exists in the "100% Free GNU/Linux distributions". I don't spend time checking these things out, honestly, but for example, all of them were shipping GLX under SGI's god-awful FreeB (and GLX) licensing at one point, well after Fedora and Debian both were aware of it. The number of things I have found (and continue to find) along with the surprise from upstreams when I inform them of the issues, forces me to draw the conclusion that the "100% Free" distributions are not doing proper audits (or if they are, they're not talking to the upstreams about it).
I found this post particularly interesting in the light of the openSUSE 11.1 release. We decided we wanted to make sure that openSUSE was freely redistributable, and get rid of the old EULA, but we don't claim that the release is "100% free" in terms of the software meeting the Free Software Foundation's definition of free, or even the Open Source Initiative's terms for being called "open source."
Not that the goal of being 100% Free isn't laudable. Just that it's not quite as easy to accomplish as it might seem at first glance.