In his article Outsourcing accessibility testing is essential, web accessibility and usability professional Peter Abrahams discusses why testing a web site for accessibility by an external third party is a good idea.

Although the article is an obvious plug for a particular testing company, the point is quite valid. Testing for web accessibility is a critical step, and a company who specializes in that usually has the best resources to do the most complete job, including testing by people with a variety of disabilities.