Here's an article from USA Today, about early tests of podcasting's effectiveness in drawing readers to publisher websites. So far, it seems to be effective. Whether the hits translate to sales is another question. People may go to the site, then leave and pick up the book on Amazon. What can publishers do to get a leg up on the trusted Amazon?
The Frog suggests amping up the added value. How about free mini-books that act as trailers for upcoming releases? Maybe 12 chapters total, each chapter from a different book and genre. This kills two birds with one stone. You encourage a sale on your site and you market your upcoming books simultaneously.