Sean Fraser from The Elementary Group Standards reveals that more than 70% of the sites participating in the most recent CSS Reboot don't validate. He provides lots of interesting details, such as the breakdown between XHTML and HTML doctypes, and whether or not the CSS validates.
After installing the HTML Validator extention for Firefox, I managed to confirm that while our upcoming Detroit Diesel redesign is valid, none of our DNN sites validate, due to the extra garbage code that DNN inserts into the rendered pages. Combine that with the horrible editor it comes with, and I'm getting less and less happy with DNN as a solution.