Pierce chicken on both sides with fork and brush both sides with salad dressing.
Spray grill rack with no stick cooking spray and arrange chicken breasts 4-5 inches over medium coals using direct heat cooking method.
Grill, covered, 5 minutes, then turn chicken and brush with salad dressing.
Continue grilling until juices run clear (3-5 minutes).
Cool and cut into thin strips (1x1/2x1/4-inch).
Cook pasta in boiling salted water (4 quarts water, 2 teaspoons salt) until just tender (12-14 minutes), drain, rinse with cold water, and drain again.
In large bowl, combine chicken, pasta, celery, green onion and grapes.
In small bowl, combine 1/4 cup oil and 1 1/4 teaspoon thyme and toss into chicken mixture.
In small bowl, combine mayonnaise, milk, honey, 1 tablespoon thyme, mustard and salt.
Refrigerate salad and dressing separately, covered, overnight.
Several hours before serving, fold dressing into salad and refrigerate, covered.
Toss walnuts into salad and spoon into lettuce lined bowl or platter to serve.