XAML: Groupfooter for GridView on Windows 8 ReleasePreview

Question

I'm trying to show a group footer in a GridView. I have found questions like How can I create a group footer in a WPF ListView ( GridView ), but they don't seem to fit anymore.

I tried by using the following ContainerStyle:

<GroupStyle.ContainerStyle>
    <Style TargetType="GroupItem">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="GroupItem">
                    <StackPanel>
                        <ContentPresenter />
                        <ItemsPresenter />
                    </StackPanel>
                </ControlTemplate>
             </Setter.Value>
         </Setter>
     </Style>
</GroupStyle.ContainerStyle>

This shows the group headers, but the items completely disappear. I'm pretty sure it's a simple miss, but I don't get it.