I am trying to set the rounded border around the selected item of the treeview (like the one in the windows explorer on Vista). The problem is that the trigger in the following code doesn't seem to be working. It looks like the IsSelected property is always false.

 <TreeView x:Name="m_treeView" BorderThickness="0" d:LayoutOverrides="Width, Height">
            <TreeView.Resources>
                <Style TargetType="TreeViewItem">
                    <Style.Resources>
                        <Brush x:Key="{x:Static SystemColors.HighlightBrushKey}">Transparent</Brush>
                    </Style.Resources>
                </Style>
            </TreeView.Resources>
            <TreeView.ItemTemplate>
                <HierarchicalDataTemplate DataType="{x:Type local:DirectoryPresentationBase}" ItemsSource="{Binding Children}">
                    <TreeViewItem>
                        <TreeViewItem.Header>
                            <Border  Name="SelectedBorder"  CornerRadius="3" Background="#EFF8FD" BorderBrush="#99DEFD"
                                     BorderThickness="1" >
                                <StackPanel Orientation="Horizontal">
                                    <interactivity:Interaction.Behaviors>
                                        <local:TreeViewExpandBehavior AssociatedTreeView="{Binding ElementName=m_treeView}"/>
                                    </interactivity:Interaction.Behaviors>
                                    <Image x:Name="img" Width="24" Height="16" Stretch="None" Source="{Binding SmallIcon}"/>
                                    <TextBlock Text="{Binding Name}" Margin="5,0" />
                                </StackPanel>
                            </Border>
                        </TreeViewItem.Header>
                    </TreeViewItem>
                    <HierarchicalDataTemplate.Triggers>
                        <Trigger Property="TreeViewItem.IsSelected" Value="True">
                            <Setter Property="Background" TargetName="SelectedBorder"  Value="Red"/>
                            <Setter Property="BorderBrush" TargetName="SelectedBorder"  Value="Green"/>
                        </Trigger>
                        <Trigger Property="TreeViewItem.IsSelected" Value="False">
                            <Setter Property="Background" TargetName="SelectedBorder"  Value="Transparent"/>
                            <Setter Property="BorderBrush" TargetName="SelectedBorder"  Value="Transparent"/>
                        </Trigger>
                    </HierarchicalDataTemplate.Triggers>
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>