Drupal and CAS utilize different logout functions. Using http://yoursite/logout will only log the user out of your site and not CAS.
Using http://yoursite/caslogout will log the user out of your site and redirect the user to the logout page for CAS.
The best method I have found to send users to the CAS logout page without hunting down and changing every script that calls logout is to setup logout as an alias for caslogout.
Setting up the Alias
- Goto: /admin/build/path/add in Drupal 6 (that's admin/config/search/path/add in Drupal 7)
- Enter "caslogout" in the Existing system path field
- Enter "logout" in the Path alias field
- Click the CREATE NEW ALIAS button.