Place chicken breast halves between two pieces of plastic wrap. Using the flat side of a meat mallet, lightly pound to an even 1/2-inch thickness. Place in a large resealable plastic bag set in a shallow dish.
For marinade, in a small bowl, stir together lime peel, lime juice, rosemary, oil, sugar, garlic, and salt. Pour over chicken. Seal bag; turn to coat chicken. Marinate in the refrigerator for 1 to 4 hours, turning bag occasionally.
Remove chicken from marinade, discarding marinade. Season chicken with pepper. For a charcoal grill, place chicken on the grill rack of an uncovered grill directly over medium coals. Grill uncovered for 10 to 12 minutes or until tender and no longer pink (170 degrees F), turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place chicken on grill rack over heat. Cover and grill as above.)