The discussion on Wikibooks is not to use License A or License B, but instead to use License A or License A & B. We do mandate that all books be released under the GFDL, we are wondering if there is the possibilities for some books to additionally cross-license with CC-BY-SA-xx. The ramifications are: 1) if the book is on Wikibooks, all future edits must be cross-licensed accordingly 2) If the book content is merged in to another book on Wikibooks that is GFDL-only, it can be taken as GFDL. 3) if an editor wants to use the book under CC-BY-SA-xx only, he would have to fork the book to another location In lieu of a project-wide licensing scheme, maybe something like a new namespace could be created that would exclusively house books that are cross-licensed? It would be trivial to change the copyright warning on the edit page to show a different licensing message in different namespaces.
Maybe we (the royal we, the WMF) need to put some pressure on the FSF to create a new GFDL version that is not so inhibitive as what we are currently using. Our copyright notice already states that content is released under "all future versions of the GFDL", so the transition would be transparent. Since WMF is one of the biggest users of the GFDL, i think we could exert that kind of pressure.
--Andrew Whitworth