Cook pasta according to package directions, omitting salt and fat. Drain.
While pasta cooks, melt margarine in a large nonstick skillet over medium-high heat. Add green onions, lemon juice, and garlic; cook 2 minutes, stirring often. Add shrimp, and cook 5 minutes or until shrimp turn pink. Remove shrimp from skillet; set aside.
Add half-and-half, cream cheese, and dill to skillet, stirring until smooth. Cook 1 to 2 minutes or until mixture is bubbly. Return shrimp to skillet, and cook until thoroughly heated. Combine shrimp mixture and pasta, tossing well. Serve immediately.