Here's a link from a radio show called The People's Pharmacy" that addresses leg cramps and the soap remedy.
http://www.peoplespharmacy.com/2005/...oap-under-the/
I have no idea why the bar soap works but apparently it is fairly successful. Looking at some of the other responses, I think success may depend on the underlying cause of the cramps. I suspect at least some of the cramps are due to electrolyte imbalance. That might explain why some remedies like pickle juice, vinegar, etc are effective. Also, very few of us are adequately hydrated so it might drinking the water that brings some relief. I've also heard about the yellow mustard remedy and see that it also contains vinegar.
Here's another cramp remedy that'll drive you nuts trying to figure out. A long distance runner told me about it and said cross-country runners often use it while running when they can't afford to stop and stretch. When you have a leg cramp, grab your entire upper lip between your thumb and index finger and pinch it HARD until the cramp eases (about 60 seconds). Make sure you grab that little flappy part (the superior labial frenulum?) on the inside of your mouth that connects your upper lip and gum. There is a nerve pressure point in that area that somehow has an effect on the muscles of the lower limbs. I can see where this would be nice in northern states when hitting a cold floor barefoot in the winter would be almost as horrible as the cramps! I rarely get leg cramps but occasionally get toe/foot cramps and have always had good results from this.