DataTemplate and Commanding

I have been quiet the last few months as I have been absorbed in a wonderful SilverLight project in the financial sector. However I am near the end and have
some free cycles. I am also working of a Windows 7 phone app and will share a few things that apply to Silverlight in general.

The first is working with SL4 Commanding and MVVM in a DataTemplated UI.Here is my template:

  <UserControl.Resources>
     local:StockServiceVM x:Key="vm"/>
   <DataTemplate x:Key="QuoteTemplate" >
     <StackPanel Name="QuoteContainer" Orientation="Vertical" Width="290" Background="Gray">
       <StackPanel Orientation="Horizontal">
          <TextBlock Foreground="White" Text="{Binding Path= Symbol, Mode=OneWay}" Width="70"/>
          <TextBlock Foreground="White" Width="78" Text="{Binding Path=Time}"/>
          <TextBlock Foreground="White" Width="80" Text="{Binding Path=Volume}"/>
          <Button x:Name="this" Width="20" HorizontalContentAlignment="Right" Content="+" 
             Command="{Binding Path=ExpandCommand,Source={StaticResource vm}}"
                  CommandParameter="{Binding ElementName= this}"></Button>
       </StackPanel>
  </DataTemplate>
</UserControl.Resources>

I am using Commanding to fire the Button click event into my viewmodel and notice the use of the Source property in the Binding definition:
Command="{Binding Path=ExpandCommand,Source={StaticResource vm}}"

If this is omitted my Binding did not work and thus my ViewModel never received the event.