mgclick, my actual experience has been that after the initial AP room discounts are offered, some dates may open up later at some resorts that were said to have fully booked all their AP rooms. Two recent examples that I had were for early Feb. 2002, when I was told that the WL was fully booked and no rooms at all were available. I phoned CRO a couple of weeks before our trip and AP room only rates were offered for the previously fully booked WL for all nights of our trip. Earlier this year I could only get six nights out of 7 nights at the AP rate, I phoned back a few days later and all seven nights were offered at the AP rates.
Don't depend on AP room rates to always open-up later, but it does happen at times.