Yep, that did the trick (my lack of knowledge driving me bananas).  Bit of refactoring to keep to the standards:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Web.Mvc;
using System.Web.Routing;
using Castle.Core;
using Castle.Core.Resource;
using Castle.Windsor;
using Castle.Windsor.Configuration.Interpreters;


namespace MySite.Web.Core
{
    public class WindsorControllerFactory : DefaultControllerFactory
    {
        private readonly WindsorContainer _container;


        public WindsorControllerFactory()
        {
            _container = new WindsorContainer(
                new XmlInterpreter(new ConfigResource("castle"))
                );
            IEnumerable<Type> controllerTypes = from t in Assembly.GetExecutingAssembly().GetTypes()
                                                where typeof (IController).IsAssignableFrom(t)
                                                select t;
            foreach (Type t in controllerTypes)
                _container.AddComponentWithLifestyle(t.FullName, t, LifestyleType.Transient);
        }


        protected override IController GetControllerInstance(RequestContext requestContext, Type controllerType)
        {
            return (IController) _container.Resolve(controllerType);
        }
    }
}