I was talking to my Dreams agent a few months ago and she was able to put my ressie into her computer for May of 2006. She explained to me that I would pay the deposit as it stands currently for this year and then as I get closer to my trip date, I would pay the difference if the rates changed. She would also apply any codes that come up as it gets closer to my trip.
I've also heard of others who have booked trips several years in advance. One poster said she booked her trip for 2008 back in 2000.