Of course it's impossible to ask everyone who has contributed to wikipedia if it's okay to dual licence their work (I'm astonished that it worked for Mozilla). But we might apply the creative commons licence to all new articles, and to articles which have been heavily edited or completely rewritten.
The problems: 1) We have to determine when an article has been sufficiently rewritten to relicense it. This can be very tricky. 2) We will not be able to import FDL content as new articles unless we specifically except it from the CC license. 3) We will not be able to import FDL content into dual licensed articles unless we revoke the CC license. 4) Our open content concept is already confusing. I can only imagine how a new visitor must feel when he reads about "dual licensing".
Regarding 2) & 3), this is the case because we cannot offer an additional choice of license for works which are not our own.
Because this will create chaos with both new and existing articles, I am against it. It is much more practical to convince someone who wants to use Wikipedia material to use the FDL than for us to effectively switch to another license. In those cases where an author absolutely wants to use CC-SA because it doesn't have the letters "GNU" before its name, they can still contact the individual authors of an article through its history, I'm sure most of them will be cooperative, if they can be contacted.
I really do like the Creative Commons idea, but I would have much preferred it if they had made use of existing licenses.
Regards,
Erik