I derailed the Flex and Ajax discussion (organized by Andre from EBA Labs) by moving it into one of my rich web application pet peeves, accessibility. What I learned is that there are some screen readers (Jaws for Windows, apparently), that actually understand enough about Flash and HTML DOM that they can actually navigate into flash and Ajax applications, and read out the contents for the visually impaired.
So, that's good, but I still feel that great strides were made in terms of accessibility with HTML, but a lot of the newer technologies for making the web richer (plugins, Java applets at first, now flash and Ajax) are taking us backwards.
-Andy.