I've been chatting with Larry Lessig about copyright and license issues, and of course he's a big proponent of his own Creative Commons licenses.
http://creativecommons.org/license/
The GNU FDL is confusing and difficult to apply in a wiki context, and a lot of the language presumes software documentation.
The Creative Commons licenses are much simpler.
I believe that the 'Attribution-Share Alike' license or the 'Share Alike' license is going to be the right one to use if we did choose to go that route.
Actually, what I think we should do, from the outset, is dual-license everything under both licenses. That ensures that the text is compatible with Wikipedia.
It's a bit late for Wikipedia proper to do much good with dual licensing, but for textbooks, it might be a good idea to do it from the outset.
--Jimbo