The module is currently being completely rewritten for Drupal 6.
The main difference is thus: instead of being a replacement of book.module (like in Drupal 5), the D6 version of outline module will depend on book.module, enhancing the already much improved book.module's capabilities.
Those who can't wait to test it may get a working copy from HEAD in the cvs repository. Use the D6 dev snapshot release for testing purposes only: the module is far from ready for use on a production site.
