Style ControlTemplate Binding not working in beta 2

I have a global Style definition in App.xaml which defines a custom ControlTemplate.  Inside the template I have a TextBlock which has it's Text bound to a dependency property (HeaderText) of the target control.  This all worked great in beta 1 but does not work now in beta 2.  Nothing jumped out at me in the breaking changes list.  Am I missing something?
<Style x:Key="DockPanelStyle" TargetType="controls:DockPanel">
<Setter Property="ButtonForeground" Value="#FF00AEEF" />
<Setter Property="Height" Value="300" />
<Setter Property="Width" Value="300" />
<Setter Property="Margin" Value="10" />
<Setter Property="HeaderTemplate">
<Setter.Value>
<ControlTemplate>
<Grid Hei ...