I added validation to an email address form field. Instead of doing the validation on the CSLA object we decided to put the validation in the viewmodel. This decision was made due to it being lower risk than involving the CSLS properties in the valid

RetailSaleAddNewCustomerView.xaml

<StackPanel Grid.Row="3" Grid.Column="1" Margin="3,0,5,0" Orientation="Vertical" HorizontalAlignment="Stretch" VerticalAlignment="Center">
        <TextBox HorizontalAlignment="Left" VerticalAlignment="Center" Text="{Binding EmailAddress,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="150" Margin="0, 5"/>
        <TextBlock Text="Email address is not valid" Foreground="Red" Visibility="{Binding DataContext.IsEmailAddressInvalidVisible,Mode=OneWay, FallbackValue='Collapsed', RelativeSource={RelativeSource FindAncestor,AncestorType={x:Type UserControl}}}"/>
</StackPanel>

RetailSaleAddNewCustomerViewModel.cs

private bool _isEmailAddressInvalid;

public bool IsEmailAddressInvalid
{
    get { return _isEmailAddressInvalid; }
    set
    {
        _isEmailAddressInvalid = value;
        RaisePropertyChanged("IsEmailAddressInvalid");
        RaisePropertyChanged("IsEmailAddressInvalidVisible");
    }
}

public Visibility IsEmailAddressInvalidVisible
{
    get { return _isEmailAddressInvalid ? Visibility.Visible : Visibility.Collapsed; }
}

public RelayCommand<object> SaveCommand
{
    get
    {
        if (_saveCommand == null)
        {
            _saveCommand = new RelayCommand<object>(
                (param) =>
                {
                    NewCustomer = NewCustomer.Save();
                    args.PersonId = NewCustomer.PersonId;

                    //Messenger.Default.Send<CloseViewEvent>(new CloseViewEvent(this, this.InstanceKey));
                    PublishEvent<CloseViewEvent, Guid>(new CloseViewEvent(this, this.InstanceKey), this.InstanceKey);
                },
                (param) =>
                {
                    return NewCustomer != null && NewCustomer.IsSavable && CustomerPhone != null && CustomerPhone.IsValid && IsEmailAddressValid(NewCustomer.EmailAddress);
                });
        }
        return _saveCommand;
    }
}

private bool IsEmailAddressValid(string emailAddress)
{
    emailAddress = emailAddress ?? String.Empty;
    var isValid = System.Text.RegularExpressions.Regex.IsMatch(emailAddress, @"^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$");
    // empty email address allowed since it is optional
    if (emailAddress == String.Empty)
    {
        isValid = true;
    }
    IsEmailAddressInvalid = !isValid;
    return isValid;
}