I actually have not been on a cruise that has been affected by a tropical storm, despite going to the Bahamas and the Caribbean on my past 2 cruises, and soon to be 3, all during hurricane season.
However, should a hurricane develop, like Jim stated, there are several possibilities. Cruise lines put SAFETY FIRST, so they will do whatever it takes to avoid the hurricane. Whether that means just changing the order of the ports, or skipping a port and having a day at sea instead.
I think if you have a day at sea instead of in a port, the cruise line should give an onboard credit to refund the port fees for the port missed.
In your case, I would keep checking the Carnival website to see if they have any updates closer to your sailing if there is a tropical system that may affect your cruise.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas