Thanx to our colleagues from PicNet, we've had support for Joomla! since CiviCRM v1.1. Joomla! is a fairly popular open source CMS and seems to have a significantly larger user base than Drupal. Unfortunately Joomla! does not make it easy to extend some core functionality like Drupal. Hence, CiviCRM on Joomla! lacks a few very important features.
We were hoping that Joomla! 1.5 would address these issues, but that did not happen. At the same time our user base on Joomla is growing quite nicely. Our download stats now indicate that the php4 version of joomla is the most downloaded tarball. The Joomla Extensions site is a close third on our referrals to civicrm.org after google and drupal. If you are a CiviCRM user on Joomla!, please do take the time to review and rate us on the Joomla extensions site.