A Shibboleth authentication module is available for Drupal 5 and 6. It can be found at http://drupal.org/project/shib_auth
When used with Drupal 6, Drupal groups membership can be generated from rules that evaluate the data contained in the SAML assertion that is provided by the identify provider. This feature is not available when using Drupal 5.
In order to use this authentication module, you will still need to install the Shibboleth SP package on your web server and you will still need to confgiure and customize the configuration. Once you have done that you will also need to configure the authentication module via the Drupal GUI.
Here is an example of the General Settings for the authentication module when used at MIT. Click on the image to enlarge it. 
Here is an example of configuring the Group Rules when using Drupal 6. Click on the image to enlarge it. 