Sorry, just not in the mood to call Disney for prices for the party. If I call, I would buy tix even if they are not in our budget. This is just the way I am...
The prices for this have been rising steadily, and there also appears to be "peak pricing" for certain times and maybe even certain days. In December it's something like $27 per adult.
I would suggest calling the dining phone number 407-939-3463. I just booked for March 27,2011, but they did not say how far out the reservations can be made. They typically add a few months at a time, not for the complete year.
You can only book over the phone. They will not have the Dessert Party if there is no Wishes scheduled for that night. You also have to pay with a credit card at the time of booking. I believe there is a 24hour cancelation policy. When you call they will give you all the details. Enjoy if you go!