An Air Canada Airbus A320-200, registration C-FNVU performing flight AC-260 from Winnipeg,MB to Toronto,ON (Canada) with 146 people on board, was climbing through FL280 out of Winnipeg when the flight crew noticed the cabin altitude was climbing and initiated an emergency descent to 10,000 feet. The cabin reached a maximum altitude of 10,500 feet, the passenger oxygen masks were not released. The aircraft returned to Winnipeg for a safe landing about 50 minutes after departure. The Canadian TSB reported the aircraft was dispatched under minimum equipment list requirements with the #2 air conditioning pack inoperative due to a cracked heat exchanger. The #1 pack became unserviceable in flight.