Just a little tip, showcasing the power of Xaml and the efficiency of good code/design separation.
When you are fetching data from a server, you could (and actually should) use the DataModel-View-ViewModel pattern, as described by Dan Crevier from the Microsoft Max team. It uses a viewmodel-class to bind your Xaml against. Dan then implements a 'state' property to indicate whether the model is finished fetching and is in a 'bindable' state.
If you are short on time, or have other reasons not to implement such a strategy, you are probably binding straight to objects that are still Null during the fetch-action.
In either case, it would be nice to let the user know that data is being fetched.
In my case, I had a lot of <TabItem>'s that are being filled with data, one-by-one, by setting the datacontext directly. While designing the user interface, I do not want to have to think about the possibility of binding against an invalid object (it being null). So I created a style, that I put in the application resources. The style replaces my content-template with a temporary template, whilst in invalid state. I was extremely satisfied with the solution, because it was so dead-easy, and it works like a charm.
Here is the code (don't mock my visually stunning border please 
):
<Style BasedOn="{StaticResource {x:Type TabItem}}" TargetType="{x:Type TabItem}">
<Style.Triggers>
<Trigger Property="DataContext" Value="{x:Null}">
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate>
<Border CornerRadius="5" BorderThickness="2" BorderBrush="Black" Background="Yellow" Height="60" Width="280"
HorizontalAlignment="Center" VerticalAlignment="Center">
<TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">Data is being fetched</TextBlock>
</Border>
</DataTemplate>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
I was pleasantly surprised by the simplicity of the code. The DataContext is used in a trigger to set the content-template.