I found this today on the Joomla! Forum and thought it was worth passing on...
A comparison of the capabilities and features of the latest releases (as of September 2008) of Joomla 1.5, Drupal 6, and Wordpress 2 with respect to 1) functionality -- multi-user publishing, layout and design, search engine optimization (SEO), mobile device support, and internationalization/localization; 2) extensibility -- the general climate and quality of third-party extension development for each platform; 3) support; 4) specific kinds of websites -- media/publishing sites, community/social sites, eCommerce sites. This is a helpful breakdown when it comes to deciding which platform is the best fit for a specific purpose.
You can view or download a PDF version of the comparison at NewLocalMedia.org.