Something I didn't state earlier as to why you have to book in advance, either at Guest Relations or when you book your cruise. The cruise line will have a list of approved passengers that have purchased transfers to Orlando Airport. You must either give your name to the representative, who will check you off the list, or in my case, hand them the "post cruise voucher" which I received with my docs since I purchased transfers when I booked the cruise.
I have to assume they're pretty strict about this, and if you're not on the list, you probably won't be allowed to board the bus.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas