Overriding BuiltinEditors.TextBoxValueTemplateKey doesn't work

I have the following code:

<UserControl>
    <UserControl.Resources>
        <DataTemplate x:Key="{x:Static p:BuiltinEditors.TextBlockNameTemplateKey}">
              ...
        </DataTemplate>

        <DataTemplate x:Key="{x:Static p:BuiltinEditors.TextBoxValueTemplateKey}">
              ...
        </DataTemplate>
    </FrameworkElement.Resources>
    <Grid>
        <p:PropertyGrid Name="pg"
                        PropertyExpandability="ForceAlways"
                        CollectionDisplayMode="EditableInline">
            <p:PropertyGrid.PropertyEditors>
                <p:PropertyEditor PropertyType="{conv:ArrayType sys:Byte}">
                    <p:PropertyEditor.ValueTemplate>
                        <DataTemplate>
                             ...
                        </DataTemplate>
                    </p:PropertyEditor.ValueTemplate>
                </p:PropertyEditor>
                <p:PropertyEditor PropertyType="{x:Type sys:Guid}">
                    <p:PropertyEditor.ValueTemplate>
                        <DataTemplate>
                             ...
                        </DataTemplate>
                    </p:PropertyEditor.ValueTemplate>
                </p:PropertyEditor>
    </Grid>
</UserControl>

 The Byte/Guid editors and TextBlockNameTemplateKey all work, but the TextBoxValueTemplateKey doesn't. Why is that?

Thanks,

Eli.