title: WP8 LongListSelector - re-assigning ItemsSource has no effect
body: <p>I'm using the new Windows Phone 8 <code>LongListSelector</code> control, which has its <code>ItemsControl</code> assigned to a <code>List&lt;Group&lt;object&gt;&gt;</code> as so:</p>
  
  <pre><code>    List&lt;Group&lt;PlacePoint&gt;&gt; searchResults; 
  
      async void doSearch()
      {
          this.searchResults = await SearchHelper.Instance.getSearchResults(txtSearchTerm.Text);
          longList.ItemsSource = this.searchResults;
      }
  </code></pre>
  
  <p>Unfortunately, the second time that I search, re-setting the .ItemsSource property has no effect and the control simply displays the old List.</p>
  
  <p>How can I change the binding?</p>