People have been very anxious for this news! Phone reservation start 8/20 and online starts 8/22! Per the Disney Parks blog this just in!

Ready, set, make your reservations beginning at noon August 20 at 407-WDW-DINE for Be Our Guest Restaurant in the New Fantasyland at Magic Kingdom Park.
You can make reservations up to 180 days out – the very first seating will be for dinner on November 19 by calling 407-WDW-DINE (3463), or visiting any Guest Relations theme park location or Guest Services desk at a resort. Online reservations will be available August 22 by visiting www.disneyworld.com/dine.