WPF TileBrush Nonsense

For the absolute life of me I cannot get a custom TileBrush to tile outside of VS2k8 or Blend 2.5. The basics are that I'm constructing a ruler, which started programmatically after finding the current DPI. When that didn't work, I attempted to do the same thing in straight XAML, which wouldn't be relative to the DPI (thus would only be a nice ruler at 96dpi). Even this would not work. At this point my frustration knows no bounds.

The above is a screengrab from inside VS2k8. The screengrab from Blend 2.5 also shows the ruler tiling. Below is a screengrab during runtime, notice the distinctive lack of tiling, and now it is somehow stretching to Fill.

The tiled brush is a DrawingBrush consisting of a GeometryDrawing with a GeometryGroup of LineGeometry's. The annotated XAML is given below:

<DrawingBrush x:Key="RulerBrush"
  Stretch="None"
  TileMode="Tile"
  Viewport="0,0,96,16"
  ViewportUnits="Absolute"
  Viewbox="0 0 1 1"
  ViewboxUnits="RelativeToBoundingBox">
<DrawingBrush.Drawing>
  <GeometryDrawing>
  <GeometryDrawing.Geometry>
     <GeometryGroup>
     <LineGeometry EndPoint="0,16"
           StartPoint="0,0" />
     <!--   ...   -->
     <LineGeometry EndPoint="48,16"
           StartPoint="48,4" />
     <!--   ...   -->
     <LineGeometry EndPoint="96,16"
           StartPoint="96,0" />
     </GeometryGroup>
  </GeometryDrawing.Geometry>
  <GeometryDrawing.Pen>
     <Pen Brush="#FF000000"
        Thickness="1" />
  </GeometryDrawing.Pen>
  </GeometryDrawing>
</DrawingBrush.Drawing>
</DrawingBrush>

And the requisite XAML for the display area:

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="Test.Window1"
  x:Name="Window"
  Title="Window"
  Width="640"
  Height="480">
  <DockPanel x:Name="LayoutRoot" LastChildFill="True">
     <ToolBarTray x:Name="ToolBarTray"
        DockPanel.Dock="Top">
        <ToolBar x:Name="MenuToolBar"
           ToolBar.Band="0"
           Width="{Binding Path=ActualWidth, ElementName=ToolBarTray}" />
     </ToolBarTray>
     <Rectangle x:Name="RulerY"
        DockPanel.Dock="Left"
        Height="15" Margin="0,15,0,0"
        Fill="{DynamicResource RulerBrush}">
        <Rectangle.LayoutTransform>
           <RotateTransform Angle="-90.0" />
        </Rectangle.LayoutTransform>
     </Rectangle>
     <Rectangle x:Name="RulerX"
        DockPanel.Dock="Top"
        Height="15" Margin="0,0,0,0"
        Fill="{DynamicResource RulerBrush}"/>
     <ScrollViewer Style="{DynamicResource SimpleScrollViewer}"/>
  </DockPanel>
</Window>

This is fairly obnoxious!