For the salad: In a small saucepan, bring water and salt to boiling. Stir in couscous. Cover and remove from heat. Let stand for 10 minutes. Transfer to a medium bowl. Cover and chill in the refrigerator.
Once chilled, stir apple, celery, walnuts and raisins into the chilled couscous.
For dressing: In a bowl, combine remaining ingredients (yogurt through pepper). Pour dressing over salad. Toss gently to combine. Chill in the refrigerator for at least one hour and up to 24 hours. Makes 6 side-dish servings.