I tried to link to this via Dare (where I originally found it) but his site was down.
MS has a project called Atlas which is designed to make it easier for anyone to build an AjAX-style web application. Of course, it will require ASP.Net 2.0 but that's awesome.
As Scott notes:
All of the pieces of AJAX – DHTML, JScript, and XMLHTTP – have been available in Internet Explorer for some time, and Outlook Web Access has used these techniques to deliver a great browser experience since 1998. In ASP.NET 2.0, we have also made it easier to write AJAX-style applications for any browser using asynchronous callbacks, and we use them in several of our built-in controls.
Can't wait - everything old is new again.