Preheat oven to 400°F. Coat grapes lightly in one tablespoon olive oil, place on foil-lined baking sheet, and roast for 10 to 12 minutes or until slightly softened. Remove from oven and reserve.
Blanch rapini in large, heavy-bottomed skillet until crisp-tender. Remove from skillet, drain, and dry.
Put remaining olive oil, garlic, and chili flakes in skillet over medium heat. Sauté until garlic is lightly browned. Add rapini and heat through. Add grapes and pine nuts and heat. Season with salt and pepper to taste.