Silverlight: Overriding a theme with a custom ItemTemplate

Question

I have a test project I created to get my feet wet with Silverlight 3. In order to get my AutoCompleteBox to display the correct property in the dropdown, I created a custom item template, like so:

<navigation:Page.Resources>
    <DataTemplate x:Key="ItemDataTemplate">
        <Grid Height="12">
            <TextBlock x:Name="TextBlock" Text="{Binding PropX, Mode=OneWay}" />
        </Grid>
    </DataTemplate>
</navigation:Page.Resources>

This worked like a charm. Then, I applied the Bureau Black theme from the November 2009 Toolkit to the layout root in the XAML (wrapping it around the LayoutRoot's definition). Now, I see the "ToString" output in the dropdown, which is the type's name, not the property, as the binding implies.

I also tried changing the foreground color of the text in my ItemTemplate, which made no difference either. How do I override the default Bureau Black ItemTemplate for the AutoCompleteBox?