Turn down service is available at all Disney deluxe resorts. The reason that it's optional, is because it's not a standard service, like daily housekeeping. At a deluxe resort, if you want nightly turn down, you can simply request it at check-in or call housekeeping from your room to arrange it. If you're staying concierge level, they will automatically give you turn down service. It usually consists of housekeeping coming to your room in the evening, between 5:15 and 9:45pm. They'll do light cleaning, as in taking out your trash, replenishing any towels or toiletries you used after daily housekeeping came, they turn down the covers on your beds and depending on which resort you're staying in, you'll get a candy and a good night poem. They will also leave a light on in your room, as well as the tv or radio. As another poster mentioned, if you have children who brought along stuffed animals, they pose them in cute positions. We've had ours reading a book in bed, or holding the tv clicker, etc.. It's a little Disney magic at the end of a long day.
As an FYI, I normally tip this service at either $1 per bed, or a $2 minimum.
Tammi