In fact, most of the airlines award seats available are based upon unsold seats predictions. With the financial crisis last year, J cabins were almost empty so many awards seats were available on most airlines, but now things are going back to normal, so I'd say the availability is going down.

I know Delta is reprocessing their awards distribution since the merger with NW, and that flights departing shortly usually have awards available, and flights departing more than a couple of weeks after the current date have almost no availability.