Hi Rob,

(I tried to post this earlier, but the proxy at work got overzealous and blocked the "post" page... Go figure).

The works well outside the MVVM context, but since I'm trying to avoid
code-behind as much as possible and also trying to make most things
work the same way across the board, I really wanted to get it to work
using EventToCommand.  Turns out I got things to work my way this
morning.

At first (after posting this question), I thought of handling
PreviewMouseDoubleClick this way:

private void xamDataTree1_PreviewMouseDoubleClick(object sender,
MouseButtonEventArgs e)
{
    DependencyObject _do = (DependencyObject)e.MouseDevice.DirectlyOver;

    XamDataTreeNodeControl node =
Infragistics.Windows.Utilities.GetAncestorFromType(_do,
                                typeof(XamDataTreeNodeControl), false)
as XamDataTreeNodeControl;

    if (node == null)
        e.Handled = true;
}

It's code-behind, but it's all UI stuff. This ensures that the
MouseDoubleClick event is triggered only when the user clicked on
something within a XamDataTreeNodeControl. The MouseDoubleClick
"EventToCommand" is then at the tree level. It kills the possibility
of double-clicking on anything else.

But this morning I found there is no need to do this. I had missed
something in the debugging output: When applying the style (with a
Blend interaction/Cinch EventToCommand), I forgot that the
XamDataTreeNodeControl doesn't deal with the same DataContext as the
tree, so it wasn't finding the command to bind to in the viewmodel. I
used a bindingproxy to resolve the binding, and now everything works
the way I want to.

I can't get into the details right now because it's a lot of stuff,
but if anyone else is interested in the solution, it's a mix of these
things:

1) BindingProxy:
http://tomlev2.wordpress.com/2011/03/21/wpf-how-to-bind-to-data-when-the-datacontext-is-not-inherited/

2) Blend interactions in a style:
http://stackoverflow.com/questions/1647815/how-to-add-a-blend-behavior-in-a-style-setter/4779168#4779168

Ultimately my XAML looks like this:

<MvvmHelpersIg:XamDataTreeEx.Resources>
    <!--  This will give the NodeLayouts in the tree access to the
tree's DataContext  -->
    <CmsCommon:BindingProxy
        x:Key="DataContextProxy"
        Data="{Binding}" />
    <MvvmHelpers:Triggers
        x:Key="MouseDoubleClickTriggers"
        x:Shared="False">
        <i:EventTrigger
            EventName="MouseDoubleClick">
            <CinchV2:EventToCommandTrigger
                Command="{Binding OpenItemEditorCommand,
                          Source={StaticResource DataContextProxy}}" />
        </i:EventTrigger>
    </MvvmHelpers:Triggers>
    <Style
        TargetType="{x:Type ig:XamDataTreeNodeControl}"
        >
        <Setter
            Property="MvvmHelpers:SupplementaryInteraction.Triggers"
            Value="{StaticResource MouseDoubleClickTriggers}" />
    </Style>
</MvvmHelpersIg:XamDataTreeEx.Resources>


Hopefully half of the above won't be chopped off after I post. 
XamDataTreeEx is a custom control that derives from XamDataTree and
supports a SelectedItem property and also the possibility to select a
node on right-click, so that can be replaced by the normal
XamDataTree.

Now I can move on to the next problem... 

Thanks,
Michel