public class CustomerViewModel
{
        private string _customerType;
        private string[] _customerTypeOptions;

        public string CustomerType
        {
            get { return _customerType; }
            set 
            {
                if (value == _customerType || string.IsNullOrEmpty(value))
                {
                    return;
                }

                _customerType = value;

                base.OnPropertyChanged("CustomerType");

            }
        }

        public string[] CustomerTypeOptions
        {
            get
            {
                if (_customerTypeOptions == null)
                {
                    _customerTypeOptions = new string[] { "(Not Specified)", "Person", "Company" };
                }

                return _customerTypeOptions;
            }
        }
}

<ComboBox Name="cmbCustomerType"
                  ItemsSource="{Binding Path=CustomerTypeOptions, Mode=OneTime}"
                  SelectedItem="{Binding Path=CustomerType, ValidatesOnDataErrors=True}" />