I was lucky enough to be able to spend some time at Burton Catalyst this last week with Pamela Dingle, looking at how to get started writing an OpenSSO plugin for Joomla to complement the plugins I recently wrote for Drupal and Wordpress. Pamela, well known for her work on PHP Information Card plugins at The Pamela Project, quickly pointed me in the right direction, and it didn't take me long after that to get something working - thanks, Pam!
As always, there is a README and source code - also available via CVS, and I've added the new provider plugin to the list on the OpenSSO Extensions page. Note that none of these plugins are supported by Sun, and all should be considered 'proof of concept' quality - they likely need a bit more polish (and lots more testing!) before being deployed into production.
So, that's the Drupal/Wordpress/Joomla open source PHP CMS trifecta covered... I see Pam has a MediaWiki plugin too - maybe I'll look at that next...