Resort Airline Check-in (RAC) is the service at WDW owned-and-operated resorts that can be used by people on "participating airlines" to check-in for their flights and check their luggage through to their final destination on the day of their flights. It is like curbside check-in, only it's at your resort rather than at the airport.
It used to be that the minimum amount of time you could use this service before your flight was 3 hours, but as of 10/15/07 it will increase to 4 hours. Since the operating hours of RAC are 5:00 a.m. to 1:00 p.m., this means that if your flight home leaves before 9:00 a.m. you will no longer be eligible to use RAC. You can still ride the Disney's Magical Express bus back to the airport, but you'd have to take your luggage to the bus (and have the driver load it underneath).
Also, RAC is only a same-day operation. In other words, you can't use RAC today to check-in for an early flight tomorrow morning.