You can only book for as far as
DCL has released dates to that point (as of today, dates extend until early Dec. 2006). However, you can still take advantage of the re-booking onboard credit or whatever promotion they are offering by booking for a date that is available and then transferring your reservation when the dates open up.
For example, we were on the Magic in January 2004 and wanted to rebook for January 2006, but the dates only went through the end of 2005 at the time (well actually the summer of 2005 was not open yet). Anyway, they were offering $200 onboard credit for the future cruise and 10% off the cruise price. The agent onboard, Donna, suggested that we book for January 2005 since the pricing would be similar and then transfer the reservation once the dates open. Just this past Wed, when 2006 opened, I transferred my reservation and they applied the $200 onboard credit and 10% off to the new booking.
If you know that you are going on a future cruise and can afford to make the deposit, I think it's worth it to make a reservation while onboard. You can always change the dates, and even if you change your mind and want to cancel, as long as you do it before the penalty period begins, you will get back your full deposit.
Good luck!