Client side Page Lifecycle with the UpdatePanel - Microsoft AJAX Library and ASP.NET 2.0 AJAX Extensions
The UpdatePanel and the asynchronous postback support a new client side
side page lifecycle. I've been going through this over the past couple
of days. Its pretty interesting stuff.
For More Info: http://ajax.asp.net/docs/tutorials/usingMsAjaxLibrary/default.aspx.