As for the dual issues...I'm just after some clarification...does Wikibooks actually stop someone from putting 'This book is licensed under the FDL and <insert license here>'? when putting a book on Wikibooks?
No, we havent ever stopped somebody from creating a book under these dual-licensing schemes. Part of it was, I think, due to our own misunderstandings of intricacies of the various licenses. To date, however, there have only been a handful of books that have been licensed in this manner. The issue has been raised most recently because a contributor at Wikibooks wants to re-license a particular book that was created from scratch on wikibooks. Since the contributor himself is the only contributor to the book (or he has received explicit permission from all other contributors). it should be possible to relicense the content. What is not certain about that situation, however, is whether wikibooks can continue to host the book if the license changes (or if the book would need to be forked to a new host).
Dual-licensing most often when people donate books to wikibooks, and want them to be released under very free licenses. Because the books tend to be mostly complete when they are donated, they are frequently not edited (much) except for spelling, grammar, and formatting errors. There is, of course, no rule that these books cannot be edited nor modified more aggressively.
If we decide that the dual-licensing of these existing books is a problem, a template could easily be employed that says "This book is released under the GFDL, but a previous revision of this book is also cross-licensed under license X" That way, we can continue development as usual, but the material can be forked from a known "safe" point.
--Andrew Whitworth