In 2007 for USA it was offered as a Bounceback to guests the end of 2006, don't remember the actual dates. They then offered it to the public in the spring. Disney VISA card holders could book first, then a few days later anyone could book it and the time to do that lasted about 7 weeks.
When we were there in 2007, there was a flyer in our room offering the Bounceback program for Sept 08 and we had to call to an in house extension to book while we were there. It was also announced on the Disney TV info channel in the room ongoing, and of course the folks here were talking about it, so if you got there and didn't see flyer you could still check with front desk or call the extension. Not all rooms received the flyer and I do not believe the front desk brought it up to checkins.
My guess is that right now you will have to wait until Disney decides if they need to offer it to the public for next year. It will probably depend on their "vacancy" rate at a certain date this spring. They will make their announcement which will hit this site (and other Disney Fan sites) pretty fast and you will know when you can call. I believe it hit the newspapers in travel sections that first week you could call, so you can keep your eye on that as well.