Last month we walked off the Magic at 8, picked up our luggage and went through the customs line by 8:30, waited a half hour for our 9am limo. We were at the airport before 10am and checked our bags and went through a very long security line and the tram and we were still waiting at our gate by 10:30 for a 12:00 flight.
It's very doable. As a previous poster mentioned, if something goes wrong -- traffic, security holdup, etc., you can run into a problem, but in my opinion that can happen anytime! Driving from the airport to Port Canaveral the day before the cruise, we were stopped on the highway for about an hour due to a car accident. We guessed that not only was it a serious accident, but ambulances and clean-up crew were apparently slowed down by the construction on the highway that left a big segment of the highway with no shoulder on either side. If that had happened on our way back to the airport, we would have probably missed our flight. If for some reason we were stuck in Orlando an extra day, we would not have missed anything urgent back home. (I would never cut it that close on the way to the cruise, we always arrive the day before!)