President Obama uses RDFa
The WhiteHouse.gov site is CC licensing all its material, and if you peek behind the covers (view source), you will see some RDFa. Granted it is a small start, but it is a start. Well done to the new administration and I look forward to more openness and more Web mashups!
Except where otherwise noted, third-party content on this site is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/3.0/us/">Creative Commons Attribution 3.0 License</a>. Visitors to this website agree to grant a non-exclusive, irrevocable, royalty-free license to the rest of the world for their submissions to <a rel="cc:attributionURL" property="cc:attributionName" xmlns:cc="http://creativecommons.org/ns#" href="http://www.whitehouse.gov">Whitehouse.gov</a> under the <a rel="license" href="http://creativecommons.org/licenses/by/3.0/us/">Creative Commons Attribution 3.0 License</a>
All the extra property, rel and xmlns attributes are used by RDFa to add additional semantic structure to the document. Above it is used to better describe the licensing for the document. This information can be used by APIs to auto-discover content licensing and determine, without human intervention, that the content is safe to use.
Learn more at the RDFa wiki.