BuddyPress.org

IANAL. But http://wiki.creativecommons.org/FAQ#Can_I_use_a_Creative_Commons_license_for_software.3F

I think (though IANAL!!) that CC licenses are generally more restrictive than the GNU under which WP is released. Because GNU licenses apply to derivative works, I think that applying a more restrictive license to a WP plugin would be a violation of the WP license.