My web design firm has been building Drupal sites for over 5 years now. Coming from a custom web design background, we've spent a lot of time evolving the basic Drupal themes from Web 2.0-ish into more stylish and professional web designs.
Below are 4 articles describing some of these accomplishments. Variations on Blue Marine shows what we could do with the most basic starting point in earlier versions of Drupal. The transparent theme effects are what we are doing now. We've also created custom Drupal templates for artists and musicians.
Since many come to us looking for "clean and corporate", this is our specialty.