I am playing with AJAX myself but I see many problems with AJAX as the next generation DHTML application platform. AJAX applications are more expensive to build, test, and update than traditional DHTML applications.
Frankly, I am not even sure whether current crop of popular web browsers can support AJAX because they weren't built with the expectation that a signle web page might stay up for as long as GUI applications. When even small carefully written DHTML apps can cause enough browser resource leaks to require frequent browser restarts, I think good stable AJAX applications will be rarer than the picture recent hype paints.
And by the time engineers discover the cost of AJAX first-hand, .NET-based ClickOnce applications will look much more attractive than AJAX-based applications can ever be.