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.