I added RDFa on a site shortly after reading that they supported it which must have been getting on for a year or so ago. I saw no discernable jumps in ranking that I could attribute to it, however I think there may be some benefits in how some search engines read, index and organise your data.
Google is using RDF data pulled from web pages in certain categories.
Do a search for a restaurant name. You will see search results from restaurant review sites with a star rating for the restaurant and the review count. Google proposed a RDF standard for websites to supply this information to Google. They offered enhanced display in the SERPs for websites that implemented it.
I believe that you need to be invited by Google to participate in this program. It isn't enough just to put the RDF data in your page.