CreativeCommons is thinking about offering a CopyRight license for wikis:
http://creativecommons.org/drafts/wiki_0.5
Annoucement at: http://www.lessig.org/blog/archives/002782.shtml
Discussion at: http://creativecommons.org/discuss#license
The wiki license is like CC by-sa except that attribution must be to the WIKI, rather to the individuals (which solves the problem of, "but if the license requires attribution, then we can't de-sign contributions on the wiki).