"B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement."
Our recommendation for third parties using Wikipedia material so far has been that it is completely sufficient in the spirit of the FDL to point to the original Wikipedia article on which the copy is based, because that page contains the history and therefore the list of *all* authors. Ulrich claims that this is not sufficient because it does not meet the conditions of modification set forth in the FDL.
Well, I think the plain language of the FDL is that Ulrich is correct. It doesn't say you can point to a list of the authors elsewhere: it says that *you* have to list the authors, and on the title page, no less (not in a list at the end of the document, for example, and *certainly* not with only a link at the end of a document to a list elsewhere). In fact, it's pretty evident that this clause of the GFDL was purposely written to disallow such things: it was written with software manuals in mind, the idea being that any third-party reproductions would be forced to proclaim loudly at the beginning who the original authors were. Saying "this is derived from [blah blah], see that book for the original authors" is clearly not intended to be sufficient.
Now whether Wikipedia would like to use the FDL is another matter. But we currently do, and changing our license seems pretty impossible at this point.
-Mark