I've worked for a number of government and non-profit clients over the years. They have always required strict ADA and accessibility compliance, but last week's ruling in a case brought against Target Corp. by the National Federation of the Blind (in Berkeley, holla!) should encourage businesses to take the regulations seriously as well. From a technical standpoint, it all comes down to writing good HTML. Little things like using the correct HTML tags for content (i.e. H1s for headers, etc.), including descriptive alternate text for images, and careful use of Flash animations can go a long way. If your content is easy to read with the technologies used by people with disabilities, it will also be more understandable to search engines. Last year Andy Hagans at A List Apart posted an article called High Accessibility Is Effective Search Engine Optimization covering the concepts in greater detail.