Beware of Joomla upgrade 1.5.16-17, not all joomla upgrades are good as we have seen from many posts on the web for versions 15.16 and 1.5.17.

Please see this posting for the exact details. 

The new Joomla release includes a auto-plugin disable feature which can lead to a site experiencing unexpected behavior or worst case the site could go down.

The feature is aimed at keeping a site up and running and not returning errors that provide clues to ways to hack components. 

To quote

"The behavior assumes that a plugin would always throw an error, being a missing file, however the error could be conditional and only thrown under certain circumstances. Depending on how the plugin works the conditions could be rare. Instead of throwing an error the plugin is disabled and prevented from executing under normal conditions. This can lead to a site visitor experiencing unexpected behavior or worse the site could go down."

joomla 1.5.18 is a good patch however. 

Joomla 1.6 has just been released and brings joomla into the enterprise CMS arena.

New features include.

New Access Control System - Allows site administrators control over who can edit, view and manage content.
Unlimited Depth Organizational Model - now user-defined category levels that allow for the creation of a category tree with as many or as few levels for organizing articles and other content as needed. No more 2 level sites.

One-Click Extension Updates - Allows users to keep sites secure and controlled by simplifying the process of updating extensions. Excellent for keeping paid for extensions up to date easily and keeping the security of the website intact. 

Semantic XHTML Layouts - Provides a better baseline for content presentation. Excellent for SEO results and better accessibility across different browser types.