In December 2007, the Board of Trustees of the Wikimedia Foundation formally decided to ask the Free Software Foundation, which administers the GNU Free Documentation License under which Wikipedia is distributed, to release a new version of the license which will allow Wikimedia to switch its content to the Creative Commons Attribution/Share-Alike license (CC-BY-SA). The underlying motivation of this change is that CC-BY-SA is an easier-to-use license granting the same essential freedoms as the GFDL. It is also more widely used by other educational projects, and switching the license would allow Wikimedia wikis to freely share content with those projects.
We’re very pleased that the Free Software Foundation has today released version 1.3 of the GNU Free Documentation License which implements this requested change. Next, the Wikimedia Foundation will organize a community wide referendum to decide whether existing GFDL wikis should be made availabe under the terms of the CC-BY-SA license.
We are deeply grateful to the Free Software Foundation for making this change. I’ve posted a more in-depth summary of what it means on the Wikimedia Foundation mailing list, and an energetic discussion on the topic has already begun. We will post more details on this topic soon.
See also:
Erik Möller
Deputy Director
The logo of the GNU project 
The Free Software Foundation (FSF) has announced a new version of the GNU Free Documentation License (GFDL) which paves the way for the Wikimedia Foundation's (WMF) projects, including the popular Wikipedia, to switch to the Creative Commons Attribution-Share Alike (CC-BY-SA) license. 
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The move was announced following discussions between the Wikimedia Foundation and the FSF.

Erik Möller, deputy director of the WMF, explained the reasoning behind this move. "The underlying motivation of this change is that CC-BY-SA is an easier-to-use license granting the same essential freedoms as the GFDL," he stated on the official Wikimedia blog. "It is also more widely used by other educational projects, and switching the license would allow Wikimedia wikis to freely share content with those projects."

Möller also state that "later this month, we will post a re-licensing proposal for all Wikimedia wikis which are currently licensed under the GFDL." 

The change was made in section eleven of the license. It says that conversion to CC-BY-SA is only possible if:
Logo of Creative Commons 
*The work is licensed under version free of the GFDL
*The work has no “Cover Texts” or “Invariant Sections.” 
*The work must have been added to a wiki, or a similar website, before November 1, 2008.

The condition of the license allows for conversion to the CC license before August 1, 2009, although after that date it will  no longer be possible. 

The Wikimedia Foundation formally decided to ask the FSF for the change in December 2007, with the approval of five of the six members of the Wikimedia board, with one member not voting.

Despite the support of the Wikimedia Foundation, some people have voiced their opposition to the move. On Advogato, Chris Frey sent an open letter to the chair of the FSF, Richard Stallman. "I am writing to express my disappointment with the Free Software Foundation regarding the recent release of the GNU Free Documentation License version 1.3," he stated. "The new version 1.3 adds a new clause, section 11, which, according to the FAQ, allows wiki sites to relicense specific content from GFDL 1.3 to CC-BY-SA 3.0, for content added before November 1, 2008. They have this relicensing option until August 1, 2009. This, in my view, is a serious moral mistake and breach of trust. Even if this new clause does no harm, it is still the wrong thing to do."