From Minh Nguyen via DotNetJunkies:

One of the most overlooked features of ASP.NET 2.0, part of Visual Studio 2005 or "Whidbey", is the Client Callback feature. This feature allows you to programmatically call server-side methods through client-side JavaScript code without the need for posting back the page. This article describes how to use the Client Callback feature to implement your own callback scenario and introduces the new TreeView control that has this feature built in.

Minh's well written article leads you through how the ASP.Net 2.0 Treeview control utilizes the new client callback system, and shows you how to develop pages, user controls, or custom controls that use this system as well.  I haven't seen much other useful information on this very promising functionality - make a point of giving this article a read.

- G