Matt Cutts, Google engineer, officially said that Google doesn't care about the W3C Validation for their SERP. Thus, another SEO myth disappeared.

He tried to explain that a valid code is welcomed, where possible, because bots (crawlers) will "digest" the information easier, but the end result does not receive any boost. He also confirmed that a page which is optimized for W3C validation, and also loads good on all browsers, would require many resources and financial losses.