I have written the following simple test in trying to learn Castle Windsor's Fluent Interface:

using NUnit.Framework;
using Castle.Windsor;
using System.Collections;
using Castle.MicroKernel.Registration;

namespace WindsorSample {
    public class MyComponent : IMyComponent {
        public MyComponent(int start_at) {
            this.Value = start_at;
        }
        public int Value { get; private set; }
    } 
    public interface IMyComponent {
        int Value { get; }
    }

    [TestFixture]
    public class ConcreteImplFixture {
        [Test]
        public void ResolvingConcreteImplShouldInitialiseValue() {
            IWindsorContainer container = new WindsorContainer();
            container.Register(Component.For<IMyComponent>().ImplementedBy<MyComponent>().Parameters(Parameter.ForKey("start_at").Eq("1")));
            IMyComponent resolvedComp = container.Resolve<IMyComponent>();
            Assert.AreEqual(resolvedComp.Value, 1); 
        }
    }
}

When I execute the test through TestDriven.NET I get the following error:

System.TypeLoadException : Could not load type 'Castle.MicroKernel.Registration.IRegistration' from assembly 'Castle.MicroKernel, Version=1.0.3.0, Culture=neutral, PublicKeyToken=407dd0808d44fbdc'.
at WindsorSample.ConcreteImplFixture.ResolvingConcreteImplShouldInitialiseValue()

When I execute the test through the NUnit GUI I get:

WindsorSample.ConcreteImplFixture.ResolvingConcreteImplShouldInitialiseValue:
System.IO.FileNotFoundException : Could not load file or assembly 'Castle.Windsor, Version=1.0.3.0, Culture=neutral, PublicKeyToken=407dd0808d44fbdc' or one of its dependencies. The system cannot find the file specified.

If I open the Assembly that I am referencing in Reflector I can see its information is:

Castle.MicroKernel, Version=1.0.3.0, Culture=neutral, PublicKeyToken=407dd0808d44fbdc

and that it definitely contains Castle.MicroKernel.Registration.IRegistration

What could be going on?

I should mention that the binaries are taken from the latest build of Castle though I have never worked with nant so I didn't bother re-compiling from source and just took the files in the bin directory. I should also point out that my project compiles with no problem.