As for re-licensing Wikipedia, personally, if it was up to me, I would be "bold" and do it the following way. I know that this is legally less secure than what Mozilla did, but I think that given our situation, it is reasonable. Someone in this thread mentioned that there is a judge who gives legal advice to Wikimedia -- maybe if he/she could post to give their opinion on whether this is legally feasible.
My idea is thus:
* Announce publicly the intended change of license from GFDL to WPL (Wikimedia Public License), and invite contributors to complain if they are unhappy with their material being re-licensed. Delete and re-write the material that is complained about.
* At some point (call it <date>), have all new articles licensed under WPL only.
* For one year starting on <date>, display a clear message on every page created before <date> stating that "This material is GFDL. We are intending to re-license it as WPL on <date+1year>. If you are the copyright holder and do not wish for your contributions to be re-licensed, please contact us."
* I imagine very few people will contact us, and we can state that people's silence is interpreted to mean they're OK with the re-licensing.
* Announce all of this extremely loudly in public. Post to all sorts of message boards, have news sites report it, etc. We have enough publicity to claim that we have contacted all contributors via public means. At that point, I believe it is no longer our responsibility if someone didn't notice anything for a whole year.
* One year after <date>, switch everything to WPL-only (or whatever you like). People may still complain after this, in which case we can still remove and re-write their work, but it is not really our responsibility that other people may have already re-used the work under WPL terms.
Timwi