Ajax wat de klok slaat, kom ik dit tegen, whidbey out of the box:

Client Callback is a new feature in ASP.NET Whidbey(2.0). It allows calling server side events asynchronously, without causing postback. It is possible to do asynchronous call using XMLHttp Post in ASP.NET 1.0 itself, but in ASP.NET 2.0 a clean framework is available to do this. This article describes how Client Callbacks works and also explores how this Client Callback framework has been implemented in ASP.NET 2.0.

http://www.extremeexperts.com/Net/Articles/ClientCallbacksinASPNETWhidbey.aspx
http://www.dotnetjunkies.com/Article/E80EC96F-1C32-4855-85AE-9E30EECF13D7.dcik
http://fredrik.nsquared2.com/viewpost.aspx?PostID=113