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. 
The entries in the screen shot above assume that you have used the MIT shell script to configure your shibboleth.xml file. You should check your xml file. Within the "Applications" section, look for the SessionInitiator section. You should have one SessionInitiator that is defined as:
<SessionInitiator id="ISDA" Location="/Touchstone" Binding="urn:mace:shibboleth:sp:1.3:SessionInit wayfURL="https://idp-mit-edu.ezproxy.canberra.edu.au/WAYF/WAYF" wayfBinding="urn:mace:shibboleth:1.0:profiles:AuthnRequest" />
The SessionInitiator shown above will direct users to the WAYF (Where Are You From) server. This is appropriate if you have an application that will support users from outside of MIT as well as users that have an MIT Kerberos username.
If your application is intended to only be accessible to users that have an MIT Kerberos username then you should have a SessionInitiator as follows:
<SessionInitiator id="MIT" Location="/MIT-only" Binding="urn:mace:shibboleth:sp:1.3:SessionInit" wayfURL="https://idp-mit-edu.ezproxy.canberra.edu.au/shibboleth-idp/SSO" wayfBinding="urn:mace:shibboleth:1.0:profiles:AuthnRequest" />
If you want to use this second SessionInitiator, then you should enter "/MIT-only" into the WAYF field in the Drupal settings page.
Here is an example of configuring the Group Rules when using Drupal 6. Click on the image to enlarge it. 