When it's officially announced, we're ALL gonna know. It'll be everywhere--Starwars.com, TheForce.net, Originaltrilogy.com, all of the different collecting sites, all of the different DVD sites, everywhere, with the official release date, and everything. It won't be from a guy named "Mainland05" who thought he saw something on some website somewhere.
In related news, I heard that they've finished Episode III early and it will start showing in an exclusive engagement at a Walla Walla, Washington theatre next week under the assumed title "Big Momma's House 3".
Take it to the bank.