Since its establishment Wikipedia has been using the GNU Free Documentation License (GFDL). The problem with this licence is that it causes incompatibility problems with the content also being released under Creative Commons licenses.
Jimmy Wales announced on Friday that the Wikimedia Foundation, the organisation behind Wikipedia, supports a solution the new version of the GFDL that will allow for integration with the CC-BY-SA licence.
Video and transcript of announcement here.
Wikimedia resolution on updating the licence here