In a medium bowl combine 6 tablespoons of the oil, vinegar, cumin, and salt and pepper to taste. Cover and store in refrigerator until chilled.
Thread scallops on skewers; brush with remaining 2 tablespoons oil. Place skewers in centre of cooking grate. Grill 6 to 8 minutes or until scallops are opaque, turning once halfway through grilling time.
Remove scallops from skewers; allow to cool. Add scallops, green onions, and tomato to chilled dressing; toss well. Serve on a bed of Bibb lettuce on individual plates.