Room rates have not been released for 2008, but you can book a room only reservation now. You will be charged one night's deposit based on 2007 rates, which will then be adjusted (up, of course!) to reflect 2008 when they are released. You will not be able to apply a
AAA discount yet (if you have AAA), but you can modify the reservation once AAA rates are released (sometime this summer).
I don't think you can book a package yet. Even though the package deposit is $200 regardless of the package cost, the packages have not been released and I don't think they will let you reserve a package yet. Someone will correct me if I am wrong, though!
If you book room-only now but want a package later, I think you have to cancel the room and book the package rather than converting the reservation from room-only to package.
Bottom-line - if you want a room-only reservation, you can book now, but the rate will change. If you want a package with tickets (with or without the Dining Plan), you will need to wait until packages are released (June or July, I think).
Happy planning!
And welcome to the DIS!