You can book a room only reservation right now. The rate you will be given is the 2008 rate. You will be required to pay a deposit equal to your first night's stay.
The last two years, the rates have been released mid-August.
If you have your heart set on a particular resort, I would suggest you go ahead and book a room only reservation. This way, you are assured you will get the resort you want.
When the 2009 rates come out, you have two choices.
1) You can keep your room only reservation and your rate will automatically be adjusted. I don't believe that you can make payments online when you book a room only reservation. I may be wrong but I believe you may have to call to make payments. ***Don't quote me on the payment part but I am sure somebody can either confirm or correct me.
2) You can call Disney reservations and ask tell them you would like to book a package. Tell them that you already have a room only reservation. They can not change the room only to a package.
They will cancel your room only reservation and then book your package. You will be refunded your room only reservation deposit. The refund is not immediate and your room only deposit can not be credited to the package price. The time delays on your room only refund depends on how you paid...i.e. credit card, debit card, etc.
You will be required to pay a $200.00 deposit (that's what it has been the last two years so I assume it will be the same for 2009) within fourteen days from the date you booked the package.
You can begin making payments on your package anytime after the deposit is made. The only requirement is that the minimum payment must be $20.00. You must pay the package off 45 days prior to your arrival date.