I just found out relatively recently myself! I also booked a courtesy reservation to make sure I could get it (it's good for 2 weeks, no deposit, deposit due on April 19, then can cancel/change up to July 31st).
The dining plan is free when you book by June 4th for a trip from August 13-Sept 30th. But I was told by the CM that rooms could fill up so you might not be able to get it if you called in early June (but she couldn't predict anything).
It quotes a value package, but you can book whatever WDW resort for however many days you want w/ whatever ticket package you want and get free dining for the dates visited. A link to the dining plan description is also on the page above. You get 1 snack, 1 table service and 1 counter service meal per person per night you're staying.
The catch is that you have to pay list price for the hotel and tickets, but the dining plan is free (normal value of $38 per adult per day!!).