According to a published heading on February 10, 2010 from the website joomla.org, Joomla powers 2.6% of the entire web. This is not such a surprising stat since it is a CMS and is quite flexible. With the new Joomla 1.6, it will be interesting to see how much committment carries on to extensions to Joomla 1.6. Here is a page that explains how to upgrade extensions:
Upgrading Joomla Extensions