title: "IsAsync" has no effect for slow property?
body: <p>I have a slow data source, so I create it asynchronously. Also, many properties of my viewmodel are themselves slow. Hence, I make them asynchronous too (binding to a Telerik property grid):</p>
  
  <pre><code>&lt;Grid&gt;
    &lt;Grid.DataContext&gt;
      &lt;ObjectDataProvider ObjectType="{x:Type viewModels:MyViewModel}"
        IsAsynchronous="True" x:Name="myViewModel" /&gt;
    &lt;/Grid.DataContext&gt;
    &lt;telerik:RadPropertyGrid x:Name="settings" Item="{Binding IsAsync=True}"&gt;
      &lt;telerik:RadPropertyGrid.PropertyDefinitions&gt;
        &lt;telerik:PropertyDefinition
          Binding="{Binding Path=SlowProperty,IsAsync=True,Mode=TwoWay}" /&gt;
      &lt;/telerik:RadPropertyGrid.PropertyDefinitions&gt;
    &lt;/telerik:RadPropertyGrid&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>When debugging, I can see that the data source is indeed created in another thread, hence not blocking the UI. However, when I get into <code>SlowProperty</code> the debugger reveals that I'm still in the Main Thread - which is proven by the fact that the UI hangs until the property returns.</p>
  
  <p><strong>Q:</strong> What am I missing here? Isn't <code>IsAsync</code> sufficient for asynchronous properties?</p>
  
  <p><strong>Edit:</strong> It seems this works for a normal <code>TextBlock</code>. Is the <code>IsAsync</code> behavior dependent on the implementation of a control? (In this case I suspect a bug in the property grid.)</p>