Combine first 6 ingredients in a large heavy-duty, zip-top plastic bag; add lamb, and seal bag. Marinate in refrigerator 12 to 24 hours.
Remove lamb from bag, reserving marinade. Place marinade in a saucepan. Bring to a boil; remove from heat. Thread lamb, pepper, tomatoes, and onion alternately onto 12 (10-inch) skewers. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place kebabs on rack; grill, covered, 5 minutes on each side or until lamb is desired degree of doneness, turning and basting kebabs frequently with marinade.