The enthusiasm over AJAX has definitely reached a fever pitch, it reminds me of the snake oil salesmen in the old west who could not find an illness their firewater wouldn’t cure. Dion does a great service in reminding people that success with AJAX is dependent on having a good foundation in place, and in understanding a few things about developing apps from services. Also worth pointing out that there are a range of tools that can be utilized to build rich clients, Flash and Flex being alternatives.
Then there’s the issue that browsers just aren’t as capable computing platforms as formal operating systems. This gives us the important constraint the creating great Ajax software still requires above average architecture, design, and development skills. As I’ve written before, Ajax isn’t for Web designers (yet), and requires some assistance from a server development team as well to build (or increasingly, find) usable Web services that will do what’s needed.