To integrate, in single sign-on, Joomla 1.5 and MediaWiki 2.1 I found an interesting plugin for joomla called "MediaWiki login" (or MediaWiki Bridge):

http://www.haroldprins.nl/index.php/mediawiki-bridge

Let's review the actors ...

• Joomla 1.5 (an existing working instance)
• MediaWiki 2.1 (a fresh instance)
• MediaWiki login (joomla plugin)
• AuthJoomla2 (mediawiki extension)

Following the simple guide provided from MediaWiki Bridge and installed the plugin and than the MW extension, however something went wrong.