As a web developer, a number of the projects I work on fall under government umbrellas and hence are subject to 508 Accessibility laws, and sometimes W3C accessibility guideline
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You might also take a look at FlashAid, although it is far from a perfect solution. (But, if you used progressive enhancement and only used AJAX when Flash was present and the user wasn't using the accessibility API, you might have a reasonable solution... for Windows.)
In the long run WAI-ARIA is the solution. It is somewhat supported in JAWS 10 (beta) and Firevox, but it certainly isn't sufficient for all of today's users.