Joomla 1.5 Importer plugin exports articles from a Joomla 1.5 database and imports them as posts into WordPress. It exports the content from a single Joomla category then allows the user to import the content into one or more WordPress categories.
This plugin does not convert Joomla articles into WordPress pages, although its framework would support such a conversion rather easily if someone would choose to code it.
This plugin does not work on Joomla 1.0 databases. If you need to convert your Joomla 1.0 data, you will first need to migrate to Joomla 1.5.
This plugin does not work on Joomla 1.6 databases. The Joomla 1.5 2-level section/category system has been replaced in Joomla 1.6 with a multi-level hierarchical system similar to WordPress. This framework could be adapted to support Joomla 1.6 in the future.
Download and info: http://wordpress.org/extend/plugins/joomla-15-importer/