<DataTemplate>                  
                  <ComboBox ItemsSource="{Binding Categories, Source={StaticResource CategoriesManager}}"
                            SelectedItem="{Binding Categories, Converter={StaticResource SelectedItemCategoriesConverter}, Mode=TwoWay}"
                            DisplayMemberPath="Name"></ComboBox>
                </DataTemplate>

 public class SelectedItemCategoriesConverter : IValueConverter
  {
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
      Categories _value = (Categories)value;
      
      if (_value != null)
      {
        _value = (from c in CategoriesManager.Categories
                  where c.CategoryID == _value.CategoryID
                  select c).First();
      }

      return _value;
    }
//....

    Private Sub dfmSettori_ContentLoaded(ByVal sender As Object, ByVal e As System.Windows.Controls.DataFormContentLoadEventArgs) Handles dfmSettori.ContentLoaded
        ' Naturalmente diventa necessario dare un nome al ComboBox all'interno del DataTemplate.
        Dim cb As ComboBox = dfmSettori.FindNameInContent("ComboBoxResponsabili")
        Dim s As ViewModel.Settore = dfmSettori.CurrentItem
        cb.SelectedItem = (From r In responsabiliColl Where r.IDSoggetto = s.Responsabile Take 1).FirstOrDefault()
    End Sub

    Private Sub ComboBoxReponsabile_DropDownClosed(ByVal sender As System.Object, ByVal e As System.EventArgs)
        Dim s As ViewModel.Settore = dfmSettori.CurrentItem
        If s Is Nothing Then _
            Return

        Dim cmb As ComboBox = sender
        s.Responsabile = DirectCast(cmb.SelectedItem, ServiceReference.Soggetto).IDSoggetto
    End Sub

                <toolkit:DataField PropertyPath="Responsabile">
                    <ComboBox x:Name="ComboBoxResponsabili" HorizontalAlignment="Stretch" ItemsSource="{Binding Source={StaticResource ResponsabiliViewSource}}" DropDownClosed="ComboBoxReponsabile_DropDownClosed">
                        <ComboBox.ItemTemplate>
                            <DataTemplate>
                                ...
                            </DataTemplate>
                        </ComboBox.ItemTemplate>
                    </ComboBox>
                </toolkit:DataField>