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I'm not sure, but I do know that the average browser usually flips from standards mode to quirks mode, if the markup at the beginning of a page does not start with the doctype - this can cause the page to be displayed wrongly.
In relation to SEO, I'm not sure, but Google uses hundreds of methods to rank it's sites and I wouldn't be surprised if one of these hundreds was having a valid code, or at least one that uses web standards - after all, Google seeks to provide it's visitors with related quality sites, and ones that conform to web standards can improve the experience of visiting a site. While this is a generalization, site's that conform to web standards, TEND to be better built, and may have a slightly higher chance of containing quality content, than sites which have been built by spammers/scammers who have no concern with web standards etc.
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