One of the topics that surfaced at CFUNITED and seems to be coming up more frequently is how AJAX-based sites deal with 508 Accessibility issues. Having just come off crutches last week, I have definitely developed a sensitivity to these issues. AJAX development presents a real challenge to the current generation of screen-reader technologies. While it's not impossible to create a fully-accessible site, it is a tough hurdle to clear.
There are ways to use javascript links and href links at the same time so that non-AJAX (and non-javascript) browsers will be able to access the same content. But this proves much more difficuly when looking at the traditional AJAX development cycle. To have an fully accessible site, each and every page must be able to be shown with it's full content. It is much easier when starting with an existing site which is already compliant, and then adding the AJAX features on top of that for some bells and whistles.
My personal feelings? AJAX is way too revolutionary for the assistive-technology vendors to ignore, and they will find a solution. That doesn't mean developers can or should ignore the issues - we should continue to make our pages 508 compliant by making sure all of our tags are complete. But once we get to the browser, everything should be standard HTML or XHTML at that point anyway, so I would imagine it wouldn't be too tough to have the screen-reader technology detect a push of content to the browser and then just read or display that content.
Want to read some more on the issue? Check out these 40 articles and tutorials on the subject from Max Kiesler's blog.