I added a TYPO3 extension to TER that enables fe_users to be automatically logged in if they're already logged in a CAS server.
The extension is implemented as a service and it extends the auth service.
Besides autologin it also includes logout functionality, so if a user logs out (by using any logout link which sets the logintype parameter to the value "logout"), the TYPO3 session is destroyed by the system, and the CAS session is destroyed by our auth service.
In this case destroying the CAS session is also required, since the autologin feature would quickly log us in after we try to destroy the session.
Download the current version straight from the TER: http://typo3.org/extensions/repository/view/rm_casautologin/current/