Combine chickpeas, onion, bread crumbs, parsley, cumin, garlic, salt and pepper in food processor. Whirl until well blended. Divide into 12 equal portions. Roll each into a ball and flatten slightly.
Coat nonstick skillet with nonstick cooking spray. Heat over medium heat. Add falafel balls; cook 2 to 3 minutes per side or until lightly browned. Set aside.
Place yogurt, tahini, lemon juice, garlic, salt and pepper in processor. Whirl until smooth. Stir in cucumber.
Split each pita bread halfway down side. Put 1/2 cup lettuce inside each pita; add 3 falafel balls and top with some of the cucumber sauce.