This works fine for me:
<Window x:Class="SO16456565.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<!--<SolidColorBrush x:Key="BG" Color="AntiqueWhite"/>-->
<x:NullExtension x:Key="BG"/>
</Window.Resources>
<Border Background="{StaticResource BG}"/>
</Window>