Guys, Peter's ladder is a win. I've had it on my roofrack for a couple of years now and regularly make good use of it - those of you who came on the river trip would have seen it; this is what it looks like.
I have teh small door ladder like JoshJ and while I don't go swinging off it, the door seems solid enough for it.
My dad made up a ladder for the Jeep roofrack that hooks on the edge of the roof rack and then has a piece at the bottom that rests on the rubber of the rear tyre. It slips under the roof rack when not in use.