Many sources including Philipp, Nathan, and Tara, have reported that Google has made an addition to their advanced search interface and now allow users to limit their search to content that carries a Creative Commons license.
+ Wording on results pages (Tara)
+ Lack of Wikipedia data (Philipp)
Btw, the Creative Commons site search interface allows you to search for content either using the Google or Yahoo database.
For more on finding Creative Commons material, see the SearchDay article: Finding Free Content in the Creative Commons.