I don't think (from what I've seen) that anyone is proposing making this retroactive. (If they were, they should know it can't be done.) As for making them GFDL from now on... I strongly vote against. A) Sometimes people will discuss things we don't have the rights to print in an article. This isn't unusl when making a point. B) GFDL is a bad fit for Wikipedia, much less mailing lists. We'd be utterly noncompliant in so many ways, it's not even funny. And once you attach a license, it's insanely difficult to get rid of it.
Now, I personally wouldn't have issue with releasing all my postings as PD, but point A would cause some trouble with that being general policy. I think a notice on the signup mail or page or something saying "You allow your statements made on this list to be used in Wikipedia." might be workable.
-- Jake