Problem is explained on StackOverflow, follow this link.

UPDATE:

The class:

public class ComboboxField : Field
    {

        public string SelectedValuePath
        {
            get { return (string)this.GetValue(SelectedValuePathProperty); }
            set { this.SetValue(SelectedValuePathProperty, value); }
        }
        public static readonly DependencyProperty SelectedValuePathProperty =
        DependencyProperty.Register("SelectedValuePath", typeof(string), typeof(ComboboxField), new PropertyMetadata(string.Empty));


        public string DisplayMemberPath
        {
            get { return (string)this.GetValue(DisplayMemberPathProperty); }
            set { this.SetValue(DisplayMemberPathProperty, value); }
        }
        public static readonly DependencyProperty DisplayMemberPathProperty =
          DependencyProperty.Register("DisplayMemberPath", typeof(string), typeof(ComboboxField), new PropertyMetadata(string.Empty, null));



        public IEnumerable ItemsSource
        {
            get { return (IEnumerable)this.GetValue(ItemsSourceProperty); }
            set { this.SetValue(ItemsSourceProperty, value); }
        }

        public static readonly DependencyProperty ItemsSourceProperty =
        DependencyProperty.Register("ItemsSource", typeof(Object), typeof(ComboboxField), new PropertyMetadata(new List<object>()));



        public object SelectedValue 
        {
            get { return (object)GetValue(SelectedValueProperty); }
            set { SetValue(SelectedValueProperty, value); }
        }
        
        public static readonly DependencyProperty SelectedValueProperty =
            DependencyProperty.Register("SelectedValue", typeof(object), typeof(ComboboxField), new PropertyMetadata(null, (s, e) => { s.SetValue(Field.ValueProperty, SelectedValueProperty);}));

        #region Ctors
        public ComboboxField(FieldDescription fieldDescription, Form parentForm) : base(fieldDescription, parentForm)
        {
            this.Template = Application.Current.Resources["ComboBoxDefaultTemplate"] as ControlTemplate;
        }

        public ComboboxField() : base(new FieldDescription(Guid.NewGuid(), "ComboboxField1", FieldType.Collection), null)
        {
            this.Template = Application.Current.Resources["ComboBoxDefaultTemplate"] as ControlTemplate;
        }
        #endregion

    }

The Control Template: (removed some of the irrelevant stuff)

<ControlTemplate x:Name="ComboBoxDefaultTemplate" TargetType="my:ComboboxField">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto" />
                    <RowDefinition Height="Auto" />
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="{TemplateBinding LabelColumnWidth}" />
                    <ColumnDefinition Width="{TemplateBinding FieldColumnWidth}" />
                </Grid.ColumnDefinitions>
                <TextBlock Grid.Row="0" Grid.Column="0" Style="{TemplateBinding TitleStyle}" Text="{TemplateBinding Title}"></TextBlock>
                <TextBlock Text="*" Grid.Row="0" Grid.Column="0" Visibility="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Required, Converter={StaticResource boolToVisibility}}" Width="5" />
                <TextBlock Grid.Row="1" Grid.Column="0" Style="{TemplateBinding SummaryStyle}" Text="{TemplateBinding Summary}" Grid.ColumnSpan="2"></TextBlock>
<ComboBox Style="{TemplateBinding FieldStyle}" Grid.Row="0" Grid.Column="1"
                          ItemsSource="{TemplateBinding ItemsSource}" 
                          SelectedValue="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedValue, Mode=TwoWay}" 
                          SelectedValuePath="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedValuePath, Mode=TwoWay}" 
                          DisplayMemberPath="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DisplayMemberPath, Mode=TwoWay}"></ComboBox>
            </Grid>
        </ControlTemplate>

The way I'm trying to use it:

<cc:ComboboxField Grid.Row="10" TitleStyle="{StaticResource labelStyle}" 
                                  Title="Countries" ItemsSource="{Binding Countries}" 
                                  SelectedValuePath="CountryId" DisplayMemberPath="CountryId" 
                                  SelectedValue="{Binding Path=SelectedCountry, Mode=TwoWay}" />

Any ideas?