Looking for information on SharePoint accessibility compliance to Section 508 of the Rehabilitation Act? Check out these links:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/odc_SP2003_ta/html/Office_SharePointAccessibility.asp
http://www.microsoft.com/industry/government/section508.mspx
It's a mixed bag - the document doesn't mention any serious conflicts with the act, but for quite a lot of features, the common answer is - "yes, this can be implemented", which implies doing a lot of site definition modification work.
Update:
Here is some details on improvements to accessibility in SharePoint 2007:
http://blogs.msdn.com/sharepoint/archive/2006/04/24/582506.aspx
