Preheat oven to 450 degree F. Carefully remove husks from corn, keeping whole. Wash husks and set aside to dry. Set aside 12 of the larger husks (about 6x4 inches). Cut corn kernels from cobs. You should have about 2 cups kernels. Discard cobs and set corn aside.
Cut polenta crosswise into 12 slices. Spray a 15x10x1-inch baking pan with nonstick coating. Lay polenta slices in single layer in pan. Bake for 15 to 20 minutes or until golden brown, turning after 10 minutes.
Meanwhile, spray a large skillet with nonstick coating. Cook corn, uncovered, over medium heat for 5 minutes or until lightly browned, stirring occasionally. Stir in tomatoes, vinegar, thyme, cumin, and salt. Cook 1 to 2 minutes. Stir in cooked shrimp; cover and heat through.
To serve, arrange 2 corn husks on individual plates. Place 2 polenta slices on corn husks; spoon the shrimp and corn mixture into husks. Garnish with cilantro. Makes 6 main-dish servings.