On juicystudio.com Joshue O` Connor published an highly interesting article on accessibility of various CMSs such like:

• Jadu
• Mambo
• Joomla
• Quick and Easy
• Expression Engine
• Plone
• Drupal
• Textpattern
• Xoops
• Typo3

Each of those CMSs was tested by an screen reader user performing some basic tasks, as well as the same tasks being performed by a user with no Assistive Technology/Special User Agent requirements and with an average computer skill level. These tests were facilitated and observed by an experienced CFIT accessibility consultant. To me absolutely unexpected Joomla! is doing a very good job. Some problems were encountered but the testers conclude: If these problems could be addressed, we would recommend Joomla. Expression Engine as well as Textpattern lack of some accessibill key features (f. e. Title and edit boxes are not marked up well and tester found it [backend] unintuitive.

Read the complete article
Choosing an Accessible CMS

anf if you like, follow the discussion on the textpattern forum.