We’ve just added the seal you see at right to Creative Commons licenses that qualify as Free Culture Licenses according to the Definition of Free Cultural Works — Attribution and Attribution-ShareAlike. Public domain is not a license, but is an acceptable copyright status for free cultural works according to the Definition.
One obvious way to think of the definition is as an application of the principles of free software to content. These demand the freedom to modify without any discrimination against uses or users, which means that Creative Commons licenses containing the NonCommercial or NoDerivatives terms do not qualify. Of course you don’t have to agree with the definition of freedom used by the free software movement and the Definition of Free Cultural Works, and even if you do agree, there may be reasons for using a more restrictive license in some cases. However, this seal and approval signals an important delineation between less and more restrictive licenses, one that creators and users of content should be aware of.
A very practical reason users should be aware of these distinctions is that some important projects accept only freely (as defined) licensed or public domain content, in particular Wikipedia and Wikimedia sites, which use the Definition of Free Cultural Works in their licensing guidelines. Indeed, clear marking of qualifying CC licenses as free is one of the issues to be addressed for a potential migration of Wikipedia to CC Attribution-ShareAlike.
This added signaling is part of an ongoing effort to distinguish among the range of Creative Commons licenses — never say the Creative Commons license, as there is no such thing. Our license deeds have always communicated the distinct properties of each license with icons and brief descriptions. In December of 2006 we added a more subtle free/less free signal — green and yellow background graphics (compare Attribution to Attribution-NonCommercial) — and began suggesting license buttons that include license property icons, so that one has an immediate visual cue as to the specific license being used without clicking through to the deed.
We hope to address further suggestions from the community and roll out further improvements in CC license deeds and the license chooser in the near future — stay tuned!