My XAML looks like this:

  <Grid x:Name="LayoutRoot">
        <dataFormToolkit:DataForm x:Name="df" CurrentItem="{Binding}" CommandButtonsVisibility="Commit,Cancel">
            <dataFormToolkit:DataForm.EditTemplate>
                <DataTemplate>
                    <StackPanel>
                        <dataFormToolkit:DataField>
                            <TextBox x:Name="txtMainProperty" Text="{Binding sMainProperty, Mode=TwoWay}"/>
                        </dataFormToolkit:DataField>
                        <dataFormToolkit:DataField>
                            <CheckBox x:Name="chkMainProperty" IsChecked="{Binding bMainProperty, Mode=TwoWay}"/>
                        </dataFormToolkit:DataField>
                        <dataFormToolkit:DataField>
                            <TextBox x:Name="txtProperty" Text="{Binding oSubEntity.sProperty, Mode=TwoWay}"/>
                        </dataFormToolkit:DataField>
                        <dataFormToolkit:DataField>
                            <CheckBox x:Name="chkProperty" IsChecked="{Binding oSubEntity.bProperty, Mode=TwoWay}"/>
                        </dataFormToolkit:DataField>
                    </StackPanel>
                </DataTemplate>
            </dataFormToolkit:DataForm.EditTemplate>
        </dataFormToolkit:DataForm>
    </Grid>

And finally, the code behind looks like this:

    public partial class MainPage : UserControl
    {
        TextBox txtProperty;
        CheckBox chkProperty;

        public MainPage()
        {
            InitializeComponent();
            this.df.ContentLoaded += new System.EventHandler(df_ContentLoaded);
        }

        private void df_ContentLoaded(object sender, DataFormContentLoadEventArgs e)
        {
            if (txtProperty != null)
            {
                txtProperty.TextChanged -= new TextChangedEventHandler(txtProperty_TextChanged);
            }
            txtProperty = this.df.FindNameInContent("txtProperty") as TextBox;
            if (txtProperty != null)
            {
                txtProperty.TextChanged += new TextChangedEventHandler(txtProperty_TextChanged);
            }

            if (chkProperty != null)
            {
                chkProperty.Click -= new System.Windows.RoutedEventHandler(chkProperty_Click);
            }
            chkProperty = this.df.FindNameInContent("chkProperty") as CheckBox;
            if (chkProperty != null)
            {
                chkProperty.Click += new System.Windows.RoutedEventHandler(chkProperty_Click);
            }
        }

        private void chkProperty_Click(object sender, System.Windows.RoutedEventArgs e)
        {
            Entity entity = this.chkProperty.DataContext as Entity;
            entity.RefreshSubEntity("bProperty", chkProperty.IsChecked);
        }

        private void txtProperty_TextChanged(object sender, TextChangedEventArgs e)
        {
            Entity entity = this.txtProperty.DataContext as Entity;
            entity.RefreshSubEntity("sProperty", txtProperty.Text);
        }
    }

This seems to work fine, but it did require the addition of a public method Entity.RefreshSubEntity(...), and new application code. I tried to treat the inner class as if it were a simple type rather than a complex object.  It is far from being ideal. No doubt there are improvements we could make in the DataForm to support this scenario out of the box, especially if the inner sub-class implemented IEditableObject as well, or a richer interface. In the meantime, this is the best I could come up with for now.

Hope it helps.