Access Matters has wrapped up an article on Today’s AJAX and DHTML Best Practices:

This article builds a sturdy three legged stool which we can stand upon to see farther. Before getting started, I want to set the stage by defining a benchmark. Accessibility’s worst case scenarios are for blind people. The blind are the hardest to accommodate. Some might think that too much attention is paid the blind when there are so many other disabilities, let alone so many more people when the wide range is counted, but the facts are that the others are easier to accommodate. Ensuring good access for the blind is the hardest challenge and therefore the benchmark.

• Part One: Making Ajax Work with Screen Readers: That article is a must read for anyone who is really interested in why all the current fuss about AJAX accessibility. Bottom line, screen readers are not yet capable of reliably recognizing when dynamically updated content has changed.
• Part Two: Ajax and Screenreaders: When Can it Work?: This article explores how various approaches work and fail (too often) with a dizzying selection of screen readers.
• Part Three: Ajax Accessibility Overview: General principles to follow

NOTE: Becky Gibson wrote the last article, and will be presenting on these ideas at The Ajax Experience!