gistfile1.cs

using System;
using Microsoft.Scripting.Hosting;

namespace InteropConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            ScriptEngine engine = IronRuby.Ruby.CreateEngine();
            ScriptScope scope = engine.CreateScope();
            engine.Execute("require 'factories.rb'", scope);
            dynamic user = engine.Execute<FactoryGirlSpike.Poco.User>("u = Factory.build(:user)", scope);
            Console.WriteLine(user);
        }
    }
}

gistfile2.builder

require 'rubygems'
require 'factory_girl'
require 'FactoryGirlSpike.dll'

include FactoryGirlSpike::Poco

# Let's define a sequence that factories can use.  This sequence defines a
# unique e-mail address.  The first address will be "somebody1@example.com",
# and the second will be "somebody2@example.com."
Factory.sequence :email do |n|
  "somebody#{n}@example.com"
end

# Let's define a factory for the User model. The class name is guessed from the
# factory name.
Factory.define :user do |f|
  # These properties are set statically, and are evaluated when the factory is
  # defined.
  f.first_name 'John'
  f.last_name  'Doe'
  f.admin      false
  # This property is set "lazily." The block will be called whenever an
  # instance is generated, and the return value of the block is used as the
  # value for the attribute.
  f.email      { Factory.next(:email) }
end

C:\Users\Will\dev\FactoryGirlSpike\InteropConsole\bin\Debug>InteropConsole.exe

Unhandled Exception: IronRuby.Classes.RuntimeError$2: Factory::DuplicateDefinitionError
   at Microsoft.Scripting.Interpreter.ThrowInstruction.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.LightLambda.Run2[T0,T1,TRet](T0 arg0, T1 arg1)
   at IronRuby.Runtime.RubyScriptCode.Run(Scope scope, Boolean bindGlobals)
   at IronRuby.Runtime.RubyScriptCode.Run(Scope scope)
   at Microsoft.Scripting.SourceUnit.Execute(Scope scope, ErrorSink errorSink)
   at Microsoft.Scripting.Hosting.ScriptSource.Execute(ScriptScope scope)
   at Microsoft.Scripting.Hosting.ScriptEngine.ExecuteFile(String path)
   at InteropConsole.Program.Main(String[] args) in C:\Users\Will\dev\FactoryGirlSpike\InteropConsole\Program.cs:line 12

Change bee0db has this working now :-)