Web Accessibility for Cognitive and Learning Disabilities: A Review of Research-Based Evidence in the Literature, by Paul R. Bohman of George Mason University, is a working paper that, at the time of this writing, was last edited July 20, 2007.
This review summarizes and critiques seven relevant studies, and has an extensive discussion on potential reasons for the paucity of relevant research. It notes common findings across studies, such as user difficulties typing text, understanding context, and navigating Web sites.
I was intrigued by the observation that Web sites could be more navigable / accessible to people with cognitive disabilities if the need for text input is reduced. I imagine one way that could be done, at least for Web site searching, would be to enable users to select from a common set of search terms, rather than require them to enter the search terms themselves.
I found enlightening the brief discussions on resistance to the idea that people with cognitive disabilities are entitled to accommodations, and on the possibility there is reluctance to accommodate people with cognitive disabilities because it may require significant Web site redesign.