There's a new Drupal Extra on http://northcountryconsulting.com. It's a variation on the project tracking system in Hour 23: Using View Arguments and Modifying Built-in Pages; this show you the bare bones of building any two-level collaborative app. This is an example of using the Drupal database as a database and not just a repository for web content.

Many people use Drupal to build websites -- and with great success. But deep inside Drupal is an SQL database (MySQL or Postgres, and in Drupal 7, SQLite). In addition to storing content in the database using the Drupal core and contributed modules, you can use CCK and Views to build a collaborative database-driven app on that same database.

The first Alpha of Drupal 7.0 is out and available for downloads. Not that the upgrade path isn't finished yet, so you can't do a test conversion of an existing site. It will happen--just not yet.

You can create a new Drupal 7 installation and see how it works (most of the highlights are in the book). And, of course, if you are developing modules or themes, the Alpha release is for you to do testing and conversion.