We support auto-login through NTLM and LDAP in the Enterprise version. Also Guest access does not require login. If you have a user that you wish to authenticate through an external system, then you'll probably want to download the source bundle and take a look at the code.
To do this, you will probably want to create your own servlet filter that intercepts page requests and performs it's own external authentication, it will also need to create the objects that the web-client needs in the users Session object to allow it to proceed into the application.
I suggest you look at the code for our authentication filter:
org.alfresco.web.app.servlet.AuthenticationFilter
and the helper that performs Guest access auto-login will probably be useful:
org.alfresco.web.app.servlet.AuthenticationHelper
Thanks,
Kevin