Trim fat from lamb. Cut lamb into 1-inch pieces; set aside.
Combine yogurt and next 7 ingredients (yogurt through garlic) in a large zip-top plastic bag. Add lamb, zucchini, tomatoes, and onion wedges; seal, and marinate in refrigerator 8 hours, turning bag occasionally. Remove lamb and vegetables from bag, reserving marinade.
Thread lamb onto 2 (10-inch) skewers. Thread zucchini, tomatoes, and onion wedges onto 2 (10-inch) skewers. Prepare grill or broiler. Place kebabs on grill rack or broiler pan coated with cooking spray; cook 7 minutes on each side or until done, basting occasionally with reserved marinade. Serve kebabs with couscous.