I was just on the Inspiration which departed Tampa on 8/25 and was supposed to return on 8/29. Well due to Hurricane Katrina we couldn't get back to Tampa so we had to go to Ft. Lauderdale. While Carnival did provide bus transporation back to Tampa, I called the airline (Song) and made arrangements to fly out of Ft. Lauderdale. I was not charged to change the tickets due to the hurricane. Some other that I spoke to did the same thing and they also were not charged. Those other airlines mentioned were JetBlue, Airtran and Delta. While they didn't have to make the change without charge I think they were very understanding of the circumstances.
Even with the hurricane in the gulf we really didn't have any bad weather. See my review on the Carnival board.