My understanding, in my very limited IFR experience, is the same as BOAC's, i.e., "straight-in" is probably a rather informal way of clearing you to the ILS without having to fly the whole procedure.
I'm answering in despite of my lack of experience because I remember requesting such an approach once, following my companion's suggestion. I was cleared to the ILS/localiser (can't remember the exact words), but not before being asked if I was visual with the ground (which I was)... I suppose that on paper ATC were still responsible for providing terrain separation but they sort of delegated that responsibility to me in the name of expeditiousness... just guessing though. I don't really know how it works.
