Google Webmaster
Answers
Google's head of the web spam division (and thus SEO), Matt Cutts answers:
"Does the crawler really care about valid HTML? "
Check out what Matt has to say.
Whether or not your site has "valid HTML" is often something you will see listed in an SEO report. While it is important for a number of reasons, finding out if you validate or not should not be a key focus of your SEO work.
As Matt said though - there are plenty of reasons you'd want to have valid HTML. WordPress core code is valid HTML - but many themes and plugins are not.