In a recent Google Webmaster Central forum thread, Google Employee John Mu was quizzed by user Nathan Sweet about whether the new HTML5 standard would provide any additional benefit to on-page SEO and in particular whether Google would pay any attention to HTML5 semantic tags. John’s response basically states that at the moment Google does not use HTML5 tags in its ranking algorithm and that until there is sufficient up-take of this new standard (and the browser support to go with it), that Google had no plans to do so stating.
“I have a feeling that HTML5 markup is not yet as widely in use (and in use correctly) that it would make sense for us to use it as a means of understanding content better. As HTML5 gains in popularity and as we recognize specific markup elements that provide value to our indexing system, this is likely to change, but at the moment I would not assume that you would have an advantage by using HTML5 instead of older variants… Personally, I would recommend using HTML5 where you think that it already makes sense, perhaps reverting to HTML4 if you can determine that the browser won’t support the elements of HTML5 that you use properly. While this will not result in an advantage for your content in our search results, it generally wouldn’t be disadvantageous either.”.
This is a strange recommendation as most web designers seek to reduce the bloat of their code however, it does highlight that as far as Google is concerned, HTML4 (and no doubt XHTML) are the way to go with regards to your on-page optimisation.