If you’re in the US, working on a US government site, or for a product that depends on a web application for any kind of authentication/access that you hope to market to the government, then understanding how WCAG 2.0 (Web Content Accessibility Guidelines) map to the existing Section 508 of the ADA (American Disabilities Act) is important. Not just running a validator or going through a check list.
There are several sites that explain how they relate, Mapping 508 to WCAG 2.0 will get you lots of results from Google (I’d list them but can only post 1 link)
Once you understand what’s required, it should be consider in every UI design since it covers things like semantics, color-contrast, Keyboard users/Assistive technology users, and so forth.
Another part is working with the content people to make sure that the content is designed/written with semantics/accessibility/usability in mind. Content Strategy is something I’d recommend reading up on and a good starting point is http://thecontentwrangler.com/2010/05/24/why-you-should-adopt-an-accessible-content-strategy/ There are several more out there.