[Zope] acl_users in plone sub folders

I'd like to take advantage of Zope's ability to have separate acl_user folders in sub folders to manage security in Plone 2.0 RC2. I'ved tried adding acl_users in a sub folder but Plone doesn't seem to recognize the users. Anyone have any ideas on if and how this can work? Does anyone know of a good solution to give certain users full access to some folders but no access to others without creating a large administrative burden?