Microsoft just released ASP.NET AJAX version 1.0 at http://ajax.asp.net! Get it while it's hot! :-) If you're new to ASP.NET AJAX and want to get started quickly, Microsoft has published several different videos. I have several here on my blog as well.
Scott Guthrie posted additional details about the release that you can read here. One of the exciting things he mentions is that both the client-side and server-side source code for ASP.NET AJAX will be released soon!