This is kind of weird.
- The Wikimedia Foundation wants to hold a vote on whether to distribute Wikipedia under a Creative Commons license, but
- All existing Wikipedia content is licensed under the CC-incompatible GNU Free Documentation License, and
- Getting permission from all the contributors to change the license would be impossible, but
- Most invocations of GNU/FSF licenses (including Wikipedia's) permit distribution under any later version of the given license, so
- The Wikimedia Foundation prevailed upon the Free Software Foundation to release a new version of the FDL specifically giving Wikipedia (and wikipedia-like “Massive Multiauthor Collaboration Sites”) a time-limited option to switch to the Creative Commons Attribution Share-Alike license.
It's a clever hack. On a practical level it seems justified given it would be impossible to relicense under any other means. On the other hand, I don't really think it's something that anyone would have had in mind when they placed their work under the FDL.
Update: While I still have the same reservations, I can understand where this move is coming from. For quite some time the GNU FDL was the only Copyleft license available that was not specific to software. As such, many people (Wikipedia included) adopted it instead of having to take the trouble to draft their own license.
This move is a recognition on the part of the FSF that the Creative Commons is a more appropriate license for many of these works. This situation is easy enough to resolve for single-author works or works where copyright was more carefully controlled. The new exception gives more complicated projects the opportunity to say “we chose the FDL by default, but if the Creative Commons had existed back then we'd have chosen that instead”.