When we first started Wikibooks I brought up the licensing issues
because I foresaw that the GFDL would limit the project and cause us
problems. Jimmy Wales (or was in Mav, or both? It's in the email
archives) stated clearly that Wikibooks would have to use the GFDL,
just like the other projects, so the issue was closed. And remains
closed.
Now that the site was started with the license it has, it is very
difficult to change, because every person who makes contributions is
doing so under an implicit acceptance of the GFDL. Each contributer
would have to give his or her permission for the license change. That
would mean getting permission from each and every person who made any
contribution, no matter how small, and even the anonymous
contributions! This would be next to impossible.
Or, throw out all of the old contributions and materials, and start
completely over with zero contributions, and a new license.
If there is going to be any dual licensing, I bet my money that it
will not be legal, and this will cause great problems for getting
books into the hands of real students. The only person who could
convince me otherwise would be a good copyright attorney, whose help
is sorely needed for this situation.
By the way do not put much (read: any) hope in the FSF being flexible
in any way whatsoever with making changes to the GFDL. Just trying to
be realistic based on past observations.