An Air Georgian Canadair CRJ-200, registration C-GOJA performing flight ZX-7376 from Toronto,ON (Canada) to Minneapolis,MN (USA), was climbing out of Toronto when flight deck and cabin began to fill with white smoke. The crew declared emergency and returned to Toronto for a safe landing on runway 05 about 12 minutes later. The Canadian TSB reported emergency services did not find any source of heat. The operator's maintenance found type IV de-icing fluid had gotten into the APU inlet during de-icing prior to takeoff causing the white smoke through the air vents.