How can I give a WPF element a rectangular flat 3D border?

Here is a solution I devised that achieves most of what I want. It doesn't give complete control over all four sides independently, but it does give the rectangular flat 3D view that I want.

Here's how it looks:

Paste this into Kaxaml to see it for yourself:

<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Background="#CCC">
  <Page.Resources>
    <!-- A brush for flat 3D panel borders -->
    <LinearGradientBrush x:Key="Flat3DBorderBrush"
                         StartPoint="0.499,0" EndPoint="0.501,1">
      <GradientStop Color="#FFF" Offset="0" />
      <GradientStop Color="#DDD" Offset="0.01" />
      <GradientStop Color="#AAA" Offset="0.99" />
      <GradientStop Color="#888" Offset="1" />
    </LinearGradientBrush>
  </Page.Resources>
  <Grid>  
    <!-- A flat 3D panel -->
    <Border
          HorizontalAlignment="Center" VerticalAlignment="Center"
          BorderBrush="{StaticResource Flat3DBorderBrush}"
          BorderThickness="1" Background="#BBB">

          <!-- some content here -->
          <Control Width="100" Height="100"/>

    </Border>  
  </Grid>
</Page>

Hope that helps someone else out. I'm still on the lookout for innovative solutions to this problem, so keep posting and I'll accept a better answer than this one.

I've done something like this just by placing multiple borders directly on top of one another. E.g.:

<Border 
  x:Name="TopAndLeft" 
  BorderThickness="3,3,0,0" 
  BorderBrush="{x:Static SystemColors.ControlLightBrush}">
<Border 
  x:Name="BottomAndRight" 
  BorderThickness="0,0,3,3" 
  BorderBrush="{x:Static SystemColors.ControlDarkBrush}">
    <ContentPresenter ... />
</Border>
</Border>

This provides the added advantage of all the other features that border provides-- corner radius and such.

Honestly probably the easiest way would be to use layering techniques. For instance create a grid like this:

  <Grid Width="50" Height="50">  
     <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
        <RowDefinition Height="Auto" />
     </Grid.RowDefinitions>
     <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="Auto" />
     </Grid.ColumnDefinitions>

     <!-- Top Border -->
     <Border Height="3" Background="LightGray" Grid.Column="0" Grid.Row="0" Grid.ColumnSpan="2" />

     <!-- Right Border -->
     <Border Width="3" Background="DarkGray" Grid.Column="2" Grid.Row="0" Grid.RowSpan="3" />

     <!-- Content -->
     <Border Background="Gray" Grid.Row="1" Grid.Column="1" />

     <!-- Left Border -->
     <Border Width="3" Background="LightGray" Grid.Row="1" Grid.Column="0" Grid.RowSpan="2" />

     <!-- Bottom Border -->
     <Border Height="3" Background="DarkGray" Grid.Row="2" Grid.Column="1" />

  </Grid>

I think you get the idea. This is probably the easiest way of doing it. You could set this up as a template and use it like this:

<Template x:Key="My3DBorder" TargetType="ContentControl">
    <!-- Put the Grid definition in here from above -->
</Template>

<ContentControl Template="{StaticResource My3dBorder}">
   <!-- My Content Goes Here -->
</ContentControl>