Joomla with extension CiviCRM.
A an extension civiCRM contact database and bulk mailing system with the Mailchimp service.
Phase I
Synch databases through XML-RPC http://www.mailchimp.com/api/how-to/sync-you-to-mailchimp.php
Period would be once daily and also trigger through manual string to sent to CiviCRM system.
1. Grab the users to be updated/created (see which above)
2. For each user, depending on their status, call:
* listSubscribe() to add new users to your List
* listUpdateMember() to update existing users on your List
* listUnsubscribe() to remove old users on your List
Ideally you would create a plug-in for Joomla http://www.mailchimp.com/plugins/
