A Delta Airlines McDonnell Douglas MD-88, registration N951DL performing flight DL-803 from Cincinnati,KY to Orlando,FL (USA) with 148 passengers, was climbing out of Northern Kentucky Airport's runway 18C when a smokey odour in the back of the cabin prompted the crew to stop the climb at 6000 feet and return to Cincinnati for a safe landing on runway 18L about 12 minutes after departure. A replacement MD-88 registration N921DL reached Orlando with a delay of 3:20 hours.