Currently, we require an "invariant section", which means that anybody has to put a specifically formatted HTML table on every page that uses Wikipedia materials, asking people to contribute to Wikipedia. (See below (*) for the rather messy details.)
Here, I want to argue that we should abandon this invariant section.
The FOLDOC computing dictionary has been licenced to us under GFDL without invariant sections. We have incorporated many articles from them. Two weeks ago, somebody asked me whether the material from our TeX article (http://www.wikipedia.com/wiki/TeX), which was originally based on FOLDOC's but has since grown considerably, could be reintegrated into FOLDOC. The answer is: only if they put our Wikipedia table into the FOLDOC entry, which they are unlikely to do because it doesn't really fit with their article formatting.
There is a new, exciting and fast growing math encyclopedia at http://planetmath.org; everything is licensed under GFDL without invariant sections, and can therefore be used by us without problems (while acknowledging the source, as we do for FOLDOC articles). I haven't copied anything over yet, but I'm sure I will in the future. People have asked me whether they could take Wikipedia materials and post them on PlanetMath. For articles that I have written exclusively myself, and there aren't many, this is no problem. For others, the Wikipedia invariant table is required, which pretty much excludes them because of the site's particular layout.
These are two examples of the fledgling open content movement that's growing right now. We are currently the clear leader of this movement, but we are not playing very nicely. If everybody required their own invariant sections, cooperation and exchange would become almost impossible. I believe that this movement is ultimately even more important than Wikipedia. We should do everything to foster it, if only out of self-interest.
Even without an invariant section, the GFDL requires proper attribution of all materials. Rather than fretting over the possible evil schemes of big bad corporations, why not apply wiki principles: trust that people are basically good, and that the more freedoms you give them, the better the outcomes will be.
Axel
---- (*) The invariant section requirement is alluded to in http://www.wikipedia.com/wiki/GNU+Free+Documentation+License, but no link is given. http://www.wikipedia.com/wiki/wikipedia:copyright contains a "draft" which explicitly disputes an invariant section. The invariant section requirement used to be contained in the uneditable file http://www.wikipedia.com/license/fdl.html but that has ceased to exist after the software change. It can still be viewed at http://web.archive.org/web/20011112090138/http://www.wikipedia.com/license/f.... The invariant sections, or "linkbacks" have been defended by Jimbo and Larry in several Wikipedia-l messages in October 2001: http://www.nupedia.com/pipermail/wikipedia-l/2001-October/date.html