AJAX or Asynchronous JavaScript and XML is by no means a new technology. I have been using this technique to prevent page refreshes and create dynamic pages for 5 years now. However, it seems that tossing a new acronym at technology makes people believe it will save the world. No wait that is XML. Alas, I'm not sure Microsoft has the right idea with the Atlas project (and they are a bit late). To me it seems the user is still stuck in an environment that was not created for a rich end user experience. I have written an article (Putting Performance into Your ASP.NET Application with Managed Client Side Controls) that proposes a work around to this problem for now. In the end, I do think Avalon's Express Applications may be the real answer (as long as we don't leave Firefox behind). I'm not crazy about another markup language like XAML, but often creating a specific language can be the best way to solve a discrete problem.
There is obviously still a place for AJAX in simple web applications that require an interactive dynamic element to improve usability.