The UK has passed, although not yet enforced, a law stating that web sites should be reasonably accessible to disabled users and in the States all government sites have to be accessible.

I am not an expert on designing accessible web sites but I feel it is an important issue and should be addressed. I personally feel that every design firm should either have an accessibility policy or make sure their designers are aware of accessiblity issues. Clients should be made aware of the fact that certain types of web design such as those that make heavy use of flash, complicated navigation systems, graphicical text or frames will most likely prevent disabled users from fully accessing the site.

One aspect of the accessibility issue that many overlook is that search engines and disabled users both require similar design considerations: html text as opposed to graphics or flash text, simple text based navigation, descriptive alt tags, etc... So if you make an effort to take accessiblity issues into account when designing sites your web site will be more search engine friendly than most and ready for optimisation.

Web Content Accessibility Guidelines 1.0
UK Disability Discrimination Act
Bobby Accessbility Web Site Checker
UseableNet Accessibility Test
Bunnyfoot - 10 Accessibility Misconceptions
Website Accessibility - Presentation by Terry Brainerd Chadwick to CHIFOO