The
DCL buses start running around 10:00am from MCO to the port. You can certainly fly in the same day as the cruise, but most people recommend flying in the day before, just in case of weather or other problems which could delay your flight (or your luggage).
You must be off the ship by, I think, 8:45am, so you can definitely fly out the same day. When we went last year, we left the ship about 8:30am, checked our luggage with Delta at the port, boarded the bus, and were at MCO by 10:00am. If you are flying out the same day, I wouldn't schedule your flight to leave before noon, in case the ship is delayed getting into port or clearing customs. You can also take the DCL bus from the port to the WDW resorts as well, if you want to stay there a few days after the cruise. IMHO, I would stay at WDW prior to the cruise, so you can go home rested and relaxed. Spending any time at WDW can be pretty exhausting.