Do not hesitate to book a cat 12 gty. It will save you some serious money, and as above, there are plenty of public areas on the ship where you will be outside. First, you save money with the cat 12. THen you save by using a Disney Visa card to pay for the trip and book with a TA who will give an onboard credit. For your next cruise, you do an onboard rebooking!
There are several dates available in 2009-10 where an adult can sail for $599 + some tax that makes it $647 per person with 2 in a cabin. $1300 for 2 people for 7 days...that's way cheaper than I can do WDW.
As others have said, other lines are cheaper, but Disney is second to none.