For the past two years, I have booked our next year's vacation on August 15th. I started calling in June of each year but had to wait until August.
You can book a room only reservation right now but your rate will change as soon as the 2008 prices come out. If you book room only, you can later change it to a package if you want.
Last week, I called 407-W-DISNEY and was told (3 different times) that the rates would be out mid-August. I just got off the phone with them and they said there was a possibility that the rates would be out the first week of July. I will definitely be calling but I have my doubts they will be out that soon.