I think Moq rocks. But it's this far from being perfect:

Dingus.cs

class Mock<T> : Moq.Mock<T> where T : class {
	public static implicit operator T(Mock<T> mock)
	{
		return mock == null ? null : mock.Object;
	}
}

class Dingus<T> : Mock<T> where T : class {
	public Dingus ()
	{
		DefaultValue = Moq.DefaultValue.Mock;
	}
}