Macromedia has recently joined forces with accessibility experts to create a product called LIFT. This product can be downloaded from www.usablenet.com/lift_dw/lift_dw.html, and it will run complete reports on your sites and assist you with making appropriate changes.
Some extensions are available for assisting you with ensuring that you are including important accessibility features in your documents. Be sure to check the Macromedia Exchange site's accessibility section for a range of utilities to assist you with the accessibility validation process.