Want to control the Joomla ACL depending on a user’s membership status?
Version 2 of the Joomla CiviCRM Membership Authentication Plugin does precisely this for Joomla 1.6 and CiviCRM 4.0 users.
Allows users to login with either their username or email address
Uses the Joomla user table to authenticate
Checks that that user has a current CiviCRM membership record
Sets the Joomla Access Level depending on the users membership status
Redirects to the configured menu item based on potential results
Also, if a user's membership has expired they can be redirected to a contribution page to renew.
More details can be found here http://wiki.civicrm.org/confluence/display/CRMDOC/Joomla+CiviCRM+Membership+Authentication+and+ACL+Plugin