IANAL. But I'm going to answer these questions from a legal viewpoint (without my personal opinion on the issue).
On Sun, 3 Aug 2003, Daniel Mayer wrote:
The question before us then is this; can we state on our Wikibooks copyright policy page and on every edit page that by pressing save, that the submitter is agreeing to grant Wikimedia a non-exclusive right to license to use their own unique and copyrightable work under both the GNU FDL /and/ any other copyleft license the Foundation may deem fit in the future (with a defintion of "copyleft" linked from that word)?
Yes.
Can authors transfer the right re-license their work through a click-through agreement like we have with the "Save page" function, under the narrowly defined terms mentioned, without assigning away all their rights to the work?
Yes.
Question two: Would such a notice prevent us from using purely FDL work (such as from Wikipedia)?
Yes. The person who is importing the work will not be able to legally save the page and meet the required conditions.
Related question: If the above is true then could we add such a notice to Wikipedia in order to cover all new submissions (we would also have to contact every current and past contributor we could in order to ask them about the change in copyright terms; if they say no or we can't find them their text will only be under the FDL)?
Yes. If we were doing that we might as well ask for copyright assignments like the FSF do, so the wikipedia will be able to defend the copyright in court if it want to.
Imran