I think the most important thing you need to know is about how authentication is performed in JBoss Portal. Authentication is performed by the servlet container.
Servlet container authentication defines several modes of authentication (BASIC,FORM,DIGEST,CLIENT_CERT), the default one is FORM which uses a form to perform login. I think that only this type of authentication is compatible with the "Remember Me" feature.
So the first for implementing this feature is to be capable to perform the login of a user transparently : i.e the user come on the web site, the portal establish trust with him (usually based on a persistent cookie that contains a proof that establish the trust) and the portal decides to perform the login of that user transparently.