Relationship Between Accessibility and Usability

The first chapter, "Understanding Web Accessibility", of Web Accessibility: Web Standards and Regulatory Compliance has some coverage the relationship between accessibility and usability.

Older information includes:

An earlier version of the "Understanding Web Accessibility" (PDF) chapter from Constructing Accessible Web Sites book is available online. See especially the first section "the Context of Accessibility: Usability".
Another –ability: Accessibility Primer for Usability Specialists is a conference paper from the Usability Professionals' Assocation Annual Conference in 2002. It discusses the relationship between accessibility and usability in product design, presents a definition of accessibility, and introduces the concept of ‘usable accessibility.

Check back later in 2006 for more on this topic...