eZ Publish, Joomla will add native support for Firebird in the future
If you look at Drupal and also Typo3, they already have multi database support.
Joomla! is currently at the beginning of developing a new Database Abstraction Layer . This new layer should also make it possible for developers to build more support for other database platforms.The new Database Abstraction Layer is not expected to be included in the current 1.5 version nor the upcoming 1.6.
If you want to read more about the new Joomla! called Joda, check there development blog.
