The DIS
site does indicate "This discount must be booked by 9/4/11."
However, the MouseForLess
site gives additional details "An Early Booking Savings will be the best discount that Passholders will receive for this travel period if booked by September 4, 2011. Discounts will not be as great for this travel period if you wait to book after September 4, 2011 (9/5 - 12/24/11). Ask for code W4K for discounts of up to 40% off, depending on when and where you stay AND how soon you book. "
Of course, my optimistic side prefers the info on the MFL site but I do concede that I've not heard about an early booking saving prior to reading this... I called earlier today (before I read this post) and still no dice for my dates/resort/room type but the CM didn't say anything about the offer expiring.