Docking Windows Phone controls to the bottom of a StackPanel

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*" />
        <RowDefinition Height="Auto" />
    <Grid.RowDefinitions>
    <StackPanel>
        <TextBlock />
        <TextBlock />
    </StackPanel>
    <UI:AdControl Grid.Row="1"/>
</Grid>