The topic of Magento integration with Joomla! has been discussed extensively on the Magento Forums and the Joomla! community. It appears the first steps of such functionality are beginning to take shape.
In a recent forum thread, Magento community member, bigodines, posted a link to a Magento and Joomla! user integration plugin (via JFusion). If you are a Joomla! junkie and would like to contribute, please give this plugin a try. Feedback is requested by the developer, so feel free to share your findings.
We are excited to see such efforts taking place and would love to see Magento and Joomla! integrated