WPF4 DataGridHeaderBorder in a xaml Style

Question

Making a separate question, related to comments on the answer to http://stackoverflow.com/questions/2481946/wpf-4-what-happened-to-datagridcolumnheader

It appears I can use DataGridHeaderBorder in a UserControl, stand-alone in a ResourceDictionary, but not in a Style's setter of a Template.

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    >

    <!-- Works -->
    <DataTemplate x:Key="yomama">
        <DataGridColumnHeader />
    </DataTemplate>

    <!-- Compile Error: error MC3074: The tag 'DataGridHeaderBorder' does not exist in XML namespace 'http://schemas.microsoft.com/winfx/2006/xaml/presentation'. -->
    <Style x:Key="{x:Type DataGridRowHeader}"
        TargetType="{x:Type DataGridRowHeader}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type DataGridRowHeader}">
                    <Grid>
                        <DataGridHeaderBorder></DataGridHeaderBorder>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

I can get it to work if I use xmlns:dg="http://schemas.microsoft.com/wpf/2008/toolkit", even though I'm not referencing WPFToolkit in the project. I've verified I'm set to .NET4 and referencing PresentationFramework v4.

Thanks for helping me remove the dg: hack.