The package discount availale to the general public had to be booked by 12/18/05.
If a discount code becomes available, you would have to mention it on the phone when you book your reservation. If you opted to book it yourself on
www.disneyworld.com, they would typically have a link to click on offering the discounted rates.
If you want to book ahead of time, and a discount comes out later down the road, you can always call back and have them try to apply it to your exsisting reservation. (I say try because they only allocate so many discounted rooms per resort.)
The Dis is a great place to keep an eye out for discount codes. If one becomes available, the news spreads like wildfire here!!

Another good site to watch is
www.mousesavers.com.
Have fun planning your trip!
