The Doom 3 source code (id tech IV) has already been released, years ago I think. They have made changes to the engine code for the BFG edition and are releasing the changes now? The BFG edition is like a Game of the Year edition, with all addons (and Doom 1 and 2 included?) right?
What is going to be different from the first source code release of id tech IV?
This is nothing major (nothing too new) I think then.