Heat margarine in a medium nonstick skillet over medium heat. Add onion, corn, and garlic, and sauté 30 seconds. Add tomato and jalapeño; sauté 4 minutes. Stir in shrimp, lemon juice, cilantro, and salt; sauté 3 minutes. Remove corn mixture from skillet; keep warm.
Place 1 tortilla in a medium nonstick skillet over medium heat; top with 1/4 cup cheese. Spoon 1/2 cup corn mixture over cheese; top with a tortilla. Cook 3 minutes pressing down with a spatula until cheese melts. Turn carefully; cook until thoroughly heated (about 1 minute). Repeat procedure with the remaining tortillas, cheese, and corn mixture. Cut each quesadilla into quarters; serve with Three-Pepper Salsa.