From the CC headquarters: "YouTube just made an incredibly exciting announcement: it’s testing an option that gives video owners the ability to allow downloads and share their work under Creative Commons licenses. ...
Many video creators on YouTube want their work to be seen far and wide. They don’t mind sharing their work, provided that they get the proper credit. Using Creative Commons licenses, we’re giving our partners and community more choices to make that happen. Creative Commons licenses permit people to reuse downloaded content under certain conditions."

Youtube blog