It is my understanding that copyright to everything in Wikipedia
belongs to the contributors, who are releasing it under the GNU FDL.
Wikipedia has a copyright on the _compilation_, which means something
very specific under copyright law, and we release the _compilation_
under the GNU FDL.
The downside of this arrangement is that if someone takes portions of
wikipedia and does something counter to the GNU FDL, we are not in a
position to defend it. That's not a good thing. I suppose that if
that were to happen, and if I were to be in the mood to defend it, we
could ask contributors to those specific sections to _at that time_
sell us their copyright for a dollar, so that we could pursue the
infringers in court.
The upside of this arrangment, if there is one, is that even Wikipedia
can't make the content proprietary.
Another potential downside to this arrangement, and I'm not sure how
important this one is, is that many overly cautious potential
licensees of the content will hesitate to license it absent a single
unified ownership. For example, Yahoo might not want to use Wikipedia
content as the basic for Encyclopedia Yahoo, if they were to create such
a thing, out of concern about there not being a single licensing authority.
--Jimbo