Via the Ease of Use mailing list, I learned of an IBM addition to the accessibility checker/assistant tools list. This one, aDesigner, looks like it could be useful in combination with other tools, heuristic evaluation and user testing (probably at the early stages). As the description states:
"The aDesigner is a disability simulator that helps Web designers ensure that their pages are accessible and usable by the visually impaired.
Voice browsers and screen readers read aloud the text on Web pages and are used by visually impaired people. However, these devices are less effective with certain kinds of content, such as highly graphical material. Web developers can use aDesigner to test the accessibility and usability of Web pages for low-vision and blind people. "
Sounds interesting and potentially useful - one to download when I get 5 minutes.