Dare Obasanjo makes a good point about the hype that is rattling around the blogosphere re: AJAX, but also SOA and REST. I find the whole AJAX thing amusing because I have seen some of our customers doing some truely amazing things around buiding compelling and responsive web applications.
The issue to date has been around tool support for the approach that AJAX promotes – I’m not really in a position to say whether AJAX solves that problem, maybe it does.
I think that the client callback feature in ASP.NET 2.0 might be a better abstraction for you if you are coming from the .NET side of the fence.