I am just curious where you came across this information, as I can find nothing on the Internet stating that Facebook is making information public as of September 17, 2012.
If you are referring to changing over to the new Timeline, your privacy settings will not change just because you switch to the new Timeline. For example, if you have your entire Facebook account set up so that "only friends" can view everything in your Facebook account, this is the way it will stay. If you desire to edit your posts, photographs, etc., and make them "public" once you have switched to the new Timeline, this is your decision.
On the other hand, if you currently have your Facebook account (or portions thereof) already set to "public," these settings will carry over to the new Timeline, but you have the option to change these settings to something else, such as "friends only," which would essentially render your Facebook account private.
To the best of my knowlege, Facebook is not taking it upon itself to make 500 billion users' account information public on September 17, 2012.
I deactivated my Facebook account, as I have absolutely no use for Facebook. However, I am still curious where you obtained this information. Would you be so kind as to let me know?