The handbook for Drupal, one of the most popular web content management systems (Spread Firefox is a recent high-profile example) has adopted Creative Commons licensing so as to

• make clear that Drupal community members can use the documentation in the Drupal handbook in other contexts, such as revising and developing site specific help docs.
• allow the integration of already existing Drupal documentation being developed outside of drupal.org by Bryght, CivicSpace, and others.
• eliminate potential legal conflicts over rights of use.

This is a good example of a community taking an existing work with a large number of contributors and obtaining the agreement necessary to take advantage of a standard Creative Commons license.

Thanks to Victor Stone for the pointer.