You could be preventing content on your site from being indexed, and you don't even know it. Release your Drupal site from evil robots right now!
A buddy at work pointed me towards a recent article that surprised me. Apparently this entire time, Drupal has been preventing content on your site from being indexed. If you upload images or any other files to your site, by default they end up somewhere in the "sites/default/files" directory on your server. If you have a Drupal powered ecommerce site with product images, they can't and won't be indexed. If you have a Drupal powered social community with tons of user generated content, anything uploaded in your 'sites' directory won't ever be found in any searches. It can be quite an issue for certain Drupal sites that have a lot of uploaded content in these directories. Open up your robots.txt file and see for yourself. If you have a Drupal website that needs the 'sites' directory's content to be crawled, unblock it now!
Here's the article explaining it in a bit more detail: http://blamcast.net/articles/drupal-seo-mistake
Update: A fix has been put in place for sites running on Drupal 6.20 or later. If you are running on 6.20 or later (7+), there are no necessary actions to take.