I wrote a small console application to test if this was possible and it turns out all you need is an XmlInclude attribute on the base type of the sub-types you are serializing.

In the following example Ninja and Customer inherit from Person, Person is decorated with the XmlInclude attribute for both Ninja and Customer.

class Program
 {
     static void Main(string[] args)
     {
         XmlSerializer serializer = new XmlSerializer(typeof(Stuff));
         Stuff stuff = new Stuff()
         {
             People = new List<Person>()
             {
                 new Person() { Name = "<b>Bob</b>", Type="Person" },
                 new Ninja() { Name = "Hiro", Type="Ninja", Weapon = "Shuriken" },
                 new Customer() { Name = "Fred", Type = "Customer", CustomerId = "1234"}
             }
         };

         serializer.Serialize(Console.Out, stuff);
     }
 }

 public class Stuff
 {
     public List<Person> People { get; set; }
 }

 [XmlInclude(typeof(Ninja)), XmlInclude(typeof(Customer))]
 public class Person
 {
     public string Type { get; set; }
     public string Name { get; set; }
 }

 public class Ninja : Person
 {
     public string Weapon { get; set; }
 }

 public class Customer : Person
 {
     public string CustomerId { get; set; }
 }