Silverlight 2.0 - vertical scrolling, horizontal flow

In silverlight 2.0. I have content that I want to scroll vertically and wrap horizontally. In the controls, I have a dock bar. The last child of the DockPanel that fills it is the ScrollViewer

<UserControl x:Class="MyProject.MyControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:WinControls="clr-namespace:Microsoft.Windows.Controls;
      assembly=Microsoft.Windows.Controls" 
    Width="400" Height="300">
    <WinControls:DockPanel LastChildFill="True">
    ...
<ScrollViewer x:Name="MessageScroll" HorizontalScrollBarVisibility="Hidden"
     VerticalScrollBarVisibility="Auto" BorderThickness="0" >
    <Controls:TextDisplay x:Name="TextDisplay"></Controls:TextDisplay>
</ScrollViewer>

      

        
        
        
      

    

The XAML TextDisplay control looks like this:

<UserControl x:Class="MyProject.TextDisplay"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
        <TextBlock x:Name="TextDisplayText" TextWrapping="Wrap">
        </TextBlock>
</UserControl>

      

        
        
        
      

    

What I want: TextDisplay should occupy the main control area, with a vertical scroll bar if the height doesn't fit. Messages should be wrapped when they get too long horizontally.

Scrolling works, but now messages don't right-wrap. they just cut it off. This doesn't limit the width, just hiding the HorizontalScrollBar. If I set HorizontalScrollBarVisibility = "Auto" I see that they scroll to the right. How do I make it wrap?