No, the only licensing detail on the Finished Maps forum is a clause in one of the stickies that states that the CG is granted a limited license to use the map in the Cartographers' Choice gallery should it be selected for that honor.

I'm still not clear on why you claim that CC are open source licenses, Redrobes. They're free, yes, but I don't see anything in there (I did read the CC BY legalese—since that's the most permissive license available, I am assuming that any relevant language would be available there) that says that sources must be available under the same license. The license runs only downstream; everything upstream of the licensed work remains available only under its original licensing terms, even if such license is exclusive to the artist. Nor do the Creative Commons licenses appear in the lists available at opensource.org (although the site itself is licensed under CC).