Add Web Reference does not create Begin.. and End.. versions of WebMethods

I've created a very simple web service (full source code below).
It works well. However, when I create a proxy in Visual Studio 2008 by using Add Web Reference, the proxy it creates doesn't have the Begin.. and End.. methods. All the documentation and web pages on this subject say that Visual Studio will generate these methods automatically.
Specifically, because my web service has only one method:
    public int Sum(int i1, int i2)
    {
        return i1 + i2;
    }
I am expecting Visual Studio to automatically generate:
    IAsyncResult BeginSum(int i1, int i2, AsyncCallback cb, object extraData)
    int EndSum(IAsyncResult asyncResult)
When I use Add Web Reference to add create a proxy for a third party web service, http://msrmaps.com/TerraService2.as ...