There has not been an official announcement about when you can start making reservations. Right now, the word is that on August 28th, ADR's will become available and that likely, the restaurant will open on Nov 19th or 20th. It is unknown if you can start making reservations for those dates or if it will be a later date. BoG may only be accepting walk ups for the "preview dates" in November until the official grand opening on Dec. 6th.
Although some are saying it may be 7am, not noon so I will be on the phone at 7am either way to be safe! Also, it was just announced that ADR's for the restaurant will start on Nov. 19th, during the preview period.
According to the Disney blog, you can also make online reservations starting 8/22/12. Though I would attempt the phone ressie on 8/20/12, if you are within the 180 day ressie period.