Kath Moonan of Abilitynet showed in her presentation user research with users with disabilities and how frustrated people can get by barriers that just are not necessary. There is a lot of good content in this one, make sure to go through all of it.
Kath Moonan's presentation at Scripting Enabled in London, September 2008. She covers the outcome of research done by Abilitynet with people with visual impairments and web sites.