Working with government clients, accessibility compliance gets raised a lot. Microsoft provides a list of Voluntary Product Accessibility Templates (VPAT) for their products. These templates describe each part of compliance and where their product may have exceptions.
MOSS is pretty well compliant out of the box. MOSS also provides a More Accessible Mode that changes the rendering and behavior of page links to be more compatible with accessibility tools such as screen readers. It’s easy to turn on, just press TAB twice after loading a page.
Of course, nobody is ever happy with OOB Sharepoint deployments, and that with where branding, customization, and custom development enter. What does all of this mean to you looking to implement a new MOSS application? If you decide to create a custom look and feel, to maintain maximum accessibility you should build from a minimal master page. That way, you can follow all the best practices like avoiding tables, placing alt tags on images, labeling your forms and all of that good stuff. Microsoft and HiSoftware have created an Accessibility kit for Sharepoint. You can also use an accessibility testing tool during development.
If you’re looking to go beyond that, ARF is a framework that emits XML from Sharepoint. You can use this and XSLT to ensure that you have valid, accessible websites and retain complete control over the entire UI.