Crumble beef into large nonstick skillet; cook over medium-high heat, stirring occasionally, for 5 to 6 minutes or until browned. Spoon off excess fat.
Add peppers, ginger, garlic and red pepper flakes to skillet. Cook over medium-high heat, stirring often, for 5 to 6 minutes or until peppers are softened.
Meanwhile, in a small bowl, whisk together cilantro, peanut butter, lime juice, soy sauce and sesame oil. Stir into beef mixture. Remove from heat.
Cut pitas in half and line insides with lettuce leaves. Fill each with about 1/2 cup of the beef mixture and serve.
Makes 4 servings.