The ValueConstraint.MaxLength needs to be data bound.  I have tried using DataBinding and it is not working, though I can use the exact same Binding statement at the same level and get the correct value.

I posted a question in the MSDN forum and they did not see anything wrong with the code, they did state that the MaxLength needs to be a dependency property of a Framework element...

 Here is the code:

<igEditors:XamTextEditor 

    Margin="8,1"
	Text="{Binding Answer, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"                         
	Visibility="{Binding ResponseParam, Converter={StaticResource EditorVisibilityConverter}, Mode=OneWay}"
	>
 
 
<!-- This does NOT work -->
    <igEditors:XamTextEditor.ValueConstraint>
	<igEditors:ValueConstraint MaxLength="{Binding ResponseParam.MaxLength, Mode=OneWay}" />
    </igEditors:XamTextEditor.ValueConstraint>
 
 
    <igEditors:XamTextEditor.ToolTip>
	<WrapPanel>
		<StackPanel Orientation="Horizontal">
			<TextBlock Text="Maximum Length: " FontWeight="Bold"/>
<!-- This works JUST fine -->
			<TextBlock Text="{Binding ResponseParam.MaxLength, Mode=OneWay}"/>
		</StackPanel>
	</WrapPanel>
    </igEditors:XamTextEditor.ToolTip>     
 
</igEditors:XamTextEditor>