This is what it says on the website:
Disney's Convenient Onboard Airline Check-In
Enjoy the comfort and convenience of Onboard Airline Check-In* when you purchase air or ground transportation through Disney Cruise Line, such as:
- Receive your boarding passes ahead of time.
- Check your luggage for your return flight before disembarking the ship.
- Skip luggage claim at the port terminal.
- Bypass the airline check-in counter at the airport.
Here's how it works:
- Reserve air or ground transportation through Disney Cruise Line, then request to sign up for Onboard Airline Check-In when you check in at the cruise port.
- On the day prior to disembarkation, your airline boarding passes and valet luggage tags will be delivered to your stateroom.
- Attach the tags to your luggage and leave your bags outside your stateroom on the evening prior to disembarkation. Your luggage will be collected and delivered to your airline for you.
- Upon arrival at Miami International Airport, you can proceed to security and continue to your departure gate.
*This service is available to Guests flying out of Miami International Airport on domestic flights (including Puerto Rico and the U.S. Virgin Islands) on AirTran Airways, American Airlines, Delta Air Lines, United Airlines and US Airways, as well as international flights on AirTran Airways, American Airlines, United Airlines and US Airways.
Pre-registration is required and scheduled flights must be after 1:30 p.m. on the day your cruise ends. Airline specific baggage fee may apply and can vary by airline. Each luggage piece can weigh no more than 50lbs. Additionally, some Guests may be selected to check in at the airport and will be unable to use this service.
So it looks like you need to have a flight later than 1:30pm to participate in the Onboard Airline Check-in from Miami. You should still be able to take transfers for earlier flights, but
DCL won't handle your luggage for you.
Enjoy your cruise!
ETA: Interestingly, this info only appears for Bahamian cruises out of Miami; Onboard Airline Check-in isn't mentioned in the information for Caribbean cruises from Miami.