cool.xaml

<ItemsControl.ItemTemplate>
    <DataTemplate>
        <StackPanel>
            <!-- TODO: to a style (outside?) -->
            <Separator x:Name="Separator"
                       Background="#1F000000"
                       Margin="0" />
            <ContentControl ContentTemplate="{StaticResource SomeTemplate}"
                            Content="{Binding}" />
        </StackPanel>
        <DataTemplate.Triggers>
            <DataTrigger Binding="{Binding RelativeSource={RelativeSource PreviousData}}"
                         Value="{x:Null}">
                <Setter Property="Visibility"
                        TargetName="Separator"
                        Value="Collapsed" />
            </DataTrigger>
        </DataTemplate.Triggers>
    </DataTemplate>
</ItemsControl.ItemTemplate>