Stir together dressing, cumin and cilantro in a large bowl. Stir in the tomato, pepper, beans and shrimp. (The mixture can be made 3 hours ahead up to this point and then covered and refrigerated.)
Arrange the shredded lettuce in a shallow serving platter. Arrange 3 cups of the tortilla chips around the outside of the platter. Crush the remaining 1 cup chips; stir into shrimp mixture. Spoon the shrimp mixture over the lettuce. Garnish with sour cream if you wish. Makes 4 servings.