In a bowl, combine mayonnaise, vinegar, honey, poppy seeds, salt and pepper. Refrigerate until ready to dress the salad. This can be prepared up to 2 days ahead.
Preheat oven to 375 degrees. Place the chicken breasts in one layer in a baking dish with 1/2 cup water. Cover with foil and bake 25 minutes or until completely cooked through. Remove cooked chicken breasts from pan, cool at room temperature for 10 minutes, then cover and refrigerate.
When chicken is cold, dice into bite-size pieces and transfer into a large bowl. Stir in nuts, grapes, celery and dressing.