You can accomplish this by setting the style of the PhoneApplicationFrame. In the App.xaml, add the following resource
<Style x:Key="AdPhoneApplicationFrameStyle" TargetType="phone:PhoneApplicationFrame">
<Setter Property="IsTabStop" Value="False"/>
<Setter Property="Foreground" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeNormal}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilyNormal}"/>
<Setter Property="HorizontalAlignment" Value="Stretch"/>
<Setter Property="VerticalAlignment" Value="Stretch"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
<Setter Property="VerticalContentAlignment" Value="Stretch"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderThickness" Value="0"/>
<Setter Property="BorderBrush" Value="{x:Null}"/>
<Setter Property="Padding" Value="0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="phone:PhoneApplicationFrame">
<Border x:Name="ClientArea" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" VerticalAlignment="{TemplateBinding VerticalAlignment}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<ContentPresenter ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
<adDuplex:AdControl Grid.Row="1"/>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
In App.xaml.cs within the InitializePhoneApplication
add the following line after the RootFrame is created
RootFrame.Style = (Style)Resources["AdPhoneApplicationFrameStyle"];
If you want to have page transitions, see this blog post for more information.