A QA Focus briefing document on "How To Evaluate A Web Site's Accessibility Level" is available.
The following references are cited in the document.
The document was written by David Sloan. David is the Project Lead of the Digital Media Access Group (DMAG) at the University of Dundee and published as How to Uncover a Web Site's Accessibility Barriers.
DMAG carry out consultancy and research in accessibility and usability issues, and have worked with many clients in the educational, public and commercial sectors in providing auditing, advice and training in accessible Web and other interface design.