When you use the UpdatePanel control you probably also want to display some kind of progress bar to the user so they can see when the async. postback starts and when it’s done, at least something that indicate that some work are being done and they need to wait until it’s done. This can be done by using the UpdateProgress control shipped with ASP.Net Ajax 1.0 Extension. If you use the UpdatePanel control and specify an AsyncPostBackTrigger and also associate the UpdateProgress to an UpdaetPanel, the UpdateProgress control will not be displayed.
Read the rest at Fredrik Normén's Blog