WPF ContextMenu woes: How do I set the DataContext of the ContextMenu?

The ContextMenu is outside of the visual tree. Below is the xaml that should get you the datacontext:

<ItemsControl ItemsSource="{Binding Markers}" Tag="{Binding ElementName=outerControl, Path=DataContext}">
   ...
   <ContextMenu DataContext="{Binding Path=PlacementTarget.Tag, RelativeSource={RelativeSource Self}}">
      <MenuItem Header="Edit"
                Command="{Binding EditCommand}" />
   </ContextMenu>
   ...
</ItemsControl>

This post explains how this works.