To the public, you should grant 'read' on the site node and you should grant 'read' on the portal_ids (member/non-member).
If applicable, you should grant 'read_private_data' on the site node to be able to view the forums portlet and you must work around the 'check whether the user is a member' in one-community.adp/tcl (if you don't want to present the non-member portal). This is how it worked on my CVS checkout (september).
I would like to see a general solution for this problem, though.
