[island911]

Not a soap, but I've had good luck with melamine sponges (Mr Clean Magic Eraser is one brand)

They are cool in that they are made of the same plastic that old ash trays and cheap plates are made of, but foamed. This makes for essentially tiny very stiff bristles. The structure does breakdown with use, but meh. Also, these sponges are effectively abrasive to the leather; being that the material is harder than the leather. --test / go lightly... Leathers have varying finishes, so it's hard to say if it will give you the results you want. My use was on one of my Cayennes. It worked beautifully, knocking back that dark sheen, returning a brighter, more matte (original) finish. All kinds of dirt trapped in the sponge.