How Important Is W3C Compliance to SEO, Really?

Found this today at SEJ. What do you think?

clipped from www.searchenginejournal.com If a site validates to the strictest of W3C validation, it is semantically correct. It’s been proven that the SEs prefer a semantically written page. It relays the information faster, cleaner, and easier to read. I’ve done testing myself on websites with identical content, one W3C valid and one not. The W3C valid site placed higher every time. In a competitive keyword situation I know my W3C valid site has a better chance at the top spot.