Hi Cheryl,
Welcome to the Moms Panel! I love to hear from guests that are all about planning ahead! It is true that the earlier you book your Disney Cruise vacation, the better price you will probably get. The exception would be if you happen to travel during their off season (September, January) because often times specials are offered during this period.
Another important reason to book early is the finite number of cabins available for each sailing. If you have a specific date that you want to sail (or have your eye on a specific cabin), the earlier you book the better because once the cabins are sold you're out of luck. Due to the popularity of cruising, along with the amazing service, style, entertainment and dining that comes with a Disney cruise, most every one pulls out of port completely booked.
Currently bookings are available through 2011, and I anticipate that the 2012 bookings will become available in the late fall of this year. So based on history the 2013 bookings will open up in late 2011.
Personally, I prefer to experience the cruise before heading over to Walt Disney World - and I recently explained why, you can read it
HERE.
I hope this helps - please let me know if you have any other planning questions!