To flatten each chicken breast half, place, boned side up, between 2 pieces of plastic wrap or waxed paper. Working from center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick.
Place chicken in shallow nonmetal dish or resealable food storage plastic bag. Add Italian salad dressing; turn to coat. Cover; refrigerate 1 hour.
Heat grill. When ready to grill, place chicken on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 8 to 10 minutes or until fork-tender and juices run clear, turning once. Toast focaccia, cut sides down, during last 3 to 4 minutes of cooking time.
In large bowl, combine lettuce, olives and Caesar salad dressing; toss to coat. Arrange cooked chicken on bottom half of toasted focaccia. Top with lettuce mixture, tomato slices and top half of focaccia. Cut into wedges to serve.