If you are using cutting edge HTML5 in your sites, you may find that the W3C validator results are incorrect.
Sitebeam puts so many requests through the validator that we have to keep a local copy on a scaling group of servers. On occasion, the W3C will update their validator, and it takes time for us to get a copy functioning on our servers. Updating the validator isn’t an automatable task, and can take anywhere from 2-10 hours. This unknown period of time needs to be scheduled in around regular development activities, and thus it can take a week or two when we become aware of a change to be able to implement a new copy on our servers.
We are working on the validator today, and will update this post when it has been resolved.
[UPDATE] We have pulled the latest available copy from the W3C, however this does not seem to have resolved the issues with RDFa. It may be the W3C has not made the latest version available yet.