Put the chicken breasts in a large pot of boiling salted water. Reduce the heat so the water just simmers and cook for 10 to 15 minutes or until done. The chicken should be white all the way through when pierced with a thin-bladed knife. Remove from the water and let cool.
Meanwhile, in a large bowl combine the yogurt, curry powder, and cayenne if using. Add the apple, raisins, celery, scallions, salt and pepper and mix again.
When the chicken is cool enough to handle, cut into 1-inch chunks and add to the bowl; toss well to combine.
Place each lettuce leaf on a plate, spoon the salad into the leaf, and serve.