.NET 4.0 Covariance

In response to another question I have tried to do the following. I don't think I interpreted that question correctly, but I do wonder if the below is possible somehow (my attempts have failed) and if not why not:

    public class MyBaseClass { }

    public class MyClass : MyBaseClass { }

    public class B<T> { }

    public class A<T> : B<T> { }

    static void Main(string[] args)
    {
        // Does not compile
        B<MyBaseClass> myVar = new A<MyClass>();
    }

I thought this might be made to work using a generic interface with a covariant type parameter:

    interface IB<out T> { }

    public class B<T> : IB<T> { }

but I was wrong about that, that does not work either.

EDIT

As SLaks has pointed out 'Interfaces are covariant; classes are not.' (thanks SLaks). So now my question is why? What was the thinking behind the design (one for Eric Lippert I think) is it not possible, undesirable or is it on a 'maybe one day' list?