1. Trim fat from lamb. Cut lamb into 1-inch pieces.
2. Combine yogurt and next 7 ingredients in a large zip-top plastic bag. Add lamb; seal bag, and marinate in refrigerator 8 hours, turning bag occasionally. Remove lamb from bag, reserving marinade.
3. Thread lamb, zucchini, tomatoes, and onion wedges alternately onto 4 (10-inch) skewers.
4. Prepare grill.
5. Place kebabs on a grill rack coated with cooking spray; grill 7 minutes on each side or until done, basting occasionally with reserved marinade. Serve kebabs with couscous.
Note: If using wooden skewers, soak them in water 30 minutes before grilling.