Server-Side Asynchronous Methods for ASP.NET and WinFX - The Code Project - C# WebServices

This article describes how to make asychronous calls to a web service. This is useful when the web service has to perform some task that takes a while to complete before returning the result to the client. By making an asynchronous call, the thread of the calling process can go back into the pool and be useful for other purposes while waiting for the response.