Right now a lot of the cruise lines have schedules going through April 2009. RCI Mariner of the Seas actually goes well into 2009.
Next Spring you'll see the balance of 2009 released (into April 2010) for most cruise lines and itineraries. It'll probably be done in what I call "fits and giggles."
Are you looking for a specific type of itnerary, ship, or cruise line?
I know Disney fans are sitting on pins and needles waiting to hear what DCL will be doing for summer 2009 ... they'll really start getting antsy by January and be full blown nightmare-ish if they have to wait until April 2008 for an announcement.