You can book a room only 499 days in advance. Rates are non-guaranteed, and they are usually pretty high- but if you're booking for a peak season, the rate you're quoted may be lower than what the actual rate will be.
If you're running in the WDW Marathon in January, you can book a Marathon Package with guaranteed rates immediately- call 407-WDW-IRUN.
When packages come out (in 2007, the 2008 packages came out in July/August) you'll be able to cancel your room only res and rebook as a package.
If you know you're coming during a busy time, or there's a specific resort/room you really want... it's definitely worth it to book the room only in advance and guarantee the availability.