TemplateBinding bug?

Question

What's the difference between
{TemplateBinding Foreground}
and
{Binding Path=Foreground, RelativeSource={RelativeSource TemplatedParent}}
?

Why does code below throw
{"Cannot convert the value in attribute 'Value' to object of type ''.  Error at object 'System.Windows.Setter'."}
?
I'm using June CTP 6.0.5456.3.0.

<Window ...>
    <Window.Resources>
        <Style TargetType="{x:Type Button}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Button}">
                        <Border Name="Border" Background="{TemplateBinding Background}"/>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsPressed" Value="true">
                                <Setter TargetName="Border" Property="Background"
                                    Value="{TemplateBinding Foreground}"/>  <!--        Here        -->
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Button Background="Green" Foreground="Red">Click Me</Button>
</Window>

Answer 1

TemplateBinding is a lightweight, but less functional, version of a Binding with a RelativeSource of TemplatedParent.  One of the things that a TemplateBinding can't do is work within the ControlTemplate.Triggers; TemplateBinding only works within the template content (for example the Border in this case).

So the {Binding} that you're using is the right solution.