C#

//.SubscribeOn(Scheduler.CurrentThread)
.ObserveOn(Scheduler.Default)

public IObservable<string> StartDownload(string uri)
      {
          WebClient wc = new WebClient();

          var o = Observable.FromEventPattern<DownloadStringCompletedEventArgs>(wc, "DownloadStringCompleted")

                            // Let's make sure that we're not on the UI Thread
                            .ObserveOn(Scheduler.Default)

                            // When the event fires, just select the string and make
                            // an IObservable<string> instead
                            .Select(newString => ProcessString(newString.EventArgs.Result));

          wc.DownloadStringAsync(new Uri(uri));

          return o;
      }

      public string ProcessString(string s)
      {
          // A very very very long computation
          Thread.Sleep(3000);
          return s + "<!-- Processing End -->";
      }

      //http://jaylee.org/post/2010/06/22/WP7Dev-Using-the-WebClient-with-Reactive-Extensions-for-Effective-Asynchronous-Downloads.aspx
      public void DisplayMyString()
      {
          var asyncDownload = StartDownload("http://bing.com");
          var asyncDownload2 = StartDownload("http://google.com");

          // Take both results and combine them when they'll be available
          var zipped = asyncDownload.Zip(asyncDownload2, (left, right) => left + " - " + right);

          // Now go back to the UI Thread
          zipped.ObserveOnDispatcher().SubscribeOnDispatcher()

                // Subscribe to the observable, and set the label text
                .Subscribe(s => label.Text = s);
      }