The Folks at Search Metrics lay down exactly what Google takes into account for SEO
We're pleasantly surprised that the poeple at Search Metrics seem to have a very detailed chart of what affects your Google listing. Of course, Google keeps their search formula secret, but extended detailed testing by SEO centric companies reveal much of what effects a site's google ranking. Check the chart out here:
We see google is still putting a huge amount of emphasis on social network mentions and backlinks to your site. Of course, Drupal takes care of a lot of the structural things needed for SEO (a lot of the minor things on that list) but the fundamental most important things to promote your site remain having backlinks to your site and having lots of talk about your site on social networking sites.