When this plugin is active and I try to access the intranet with a normal browser, I don't get the login form I'd expect. My browser does not support SPNEGO and Plone should show me the cookie plugin's login_form.
If I try to visit the intranet with a browser that is SPNEGO enabled (ie. firefox with 'network.negotiate-auth.delegation-uris' and 'network.negotiate-auth.trusted-uris' set), I get a login for because I'm not logged in (on my machine) as a domain user.