static List<RadTreeViewItem> SelectedItemsX(this RadTreeViewItem treeView)
{
    if (treeView == null) throw new ArgumentNullException();
    var treeViewItemList = new List<RadTreeViewItem>();
    var list = treeView.ItemsSource as IEnumerable;
    if (list == null)
        return treeViewItemList;
    foreach (var item in list)
    {
        var itemFound = treeView.ParentTreeView.ContainerFromItemRecursive(item);
//IsSelected here never seems to return true
        if (itemFound != null && itemFound.IsSelected == true)
            treeViewItemList.Add(itemFound);
    }
    return treeViewItemList;
}

Looks like using IsSelected is for node selections rather than checking checkbox items.  Replacing the IsSelected line with this:

if (itemFound != null && itemFound.CheckState == System.Windows.Automation.ToggleState.On)

Works but doesn't seem incredibly efficient because I need to call that recursive function for each item.  Any ideas?  Thanks.

-jo