Ajax is a concept who's time has finally come to the mainstream. We now have browsers which support enough of the web standards to be able to support richer web interfaces.
Marc Logemann argues that rich web applications would be better architected as client side applications in Swing or C#. I think the use of client side applications has slowed as we shift intelligence and CPU cycles outward. Not every application is suited to live on the server side but building richer interfaces to web sites or web applications fills in the middle ground between client side and server side.
Round trip delays have always prevented web applications from rivaling client side applications. The ease of an instant on always updated application is the appeal in using a web application. By leveraging Ajax concepts we can provide the user with a much more responsive experience.