Very few cruise ships on any line are bookable past April 2010. Prices comes out in April or May for cruises sailing until about two years later. In April or May of this year, you should be able to book ships sailing through April 2011. Most cruise lines won't confirm itineraries for that time period until the new prices come out.
There are some exceptions. For example, Royal Caribbean is allowing bookings for their new Oasis of the Seas past April 2010. But in general, that's how it works.