December 31 and January 1st will book up pretty quickly as many are traveling to WDW during Christmas week.
You can, actually, book your Magic Your Way package now for dates starting in Dec 08 and traveling into 2009. That would be my suggstion. However, Disney does reserve the right to change the final package amount based on their 2009 package rates (usually released mid-late summer).
It is not often that there are specials for December 31st, but should one come out for arrivals on January 1 (such as right now, there is a free upgrade ticket special) you can then just cancel the first night of your reserved package and have it re-rated at the discount. Then, you can simply book a room-only reservation for the 31st.
I hope that helps and wasn't too confusing.
