The Carnival site only has cruise listings through May 2009. And is typical for them to be about 18 months out as Dave noted. I don't know the reason they are listed on other sites, and not the main site. Unless maybe there could be changes, and they don't want to mass advertise the dates and rates themselves. I am just guessing here.
Neil, I like the site for search purposes you (I assume) are refering to. I appreciate your discretion.