Rx.cs

var request = WebRequest.Create("http://github.com/");
var html = from response in request.GetResponseAsync()
           let responseStream = response.GetResponseStream()
           let reader = new StreamReader(responseStream)
           select reader.ReadToEnd();

var htmlSub = html.Subscribe(Console.WriteLine);

public static IObservable<WebResponse> GetResponseAsync(this WebRequest request)
{        
    return Observable.FromAsyncPattern(request.BeginGetResponse, ar => request.EndGetResponse(ar))();
}