The question of having valid HTML and would it help your rankings in Google or other search engines is not new. Most SEOs believed that it made no impact on your rankings, unless Googlebot has serious issues crawling your site.
In a recent YouTube video, Google's Matt Cutts explained why Google.com does not validate (historically has not) and also added that Google doesn't "give any sort of boost to web pages that validate." He explained because "the vast majority of pages on the web don't validate," including Google.com.
Here is the video:
For those in the SEO community and know Edward Lewis, aka pageoneresults, you will find it funny that he posted that thread at WebmasterWorld.
Forum discussion at WebmasterWorld.