two things :
there is this :
Making a LoginPortlet is not recommended
a lot of problems on the way
see : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4091965#4091965
second thing :
for the start page going directly to the login page :
there are post about that
if all your portal page are "authentification needed", then, at any visit, the user will first be redirected to the login page, for authentication...
hope it helps...