As promised, id Software has published the source code of Doom 3. Developers interested in playing with the code can get it over here.
This source release does not contain any game data, the game data is still
covered by the original EULA and must be obeyed as usual.
You must patch the game to the latest version.
The publication of the source code was delayed due to legal hurdles because a shading algorithm technique patent used in Doom 3 is owned by Creative Labs. Therefore, id Software had to use a different preload technique, as described over here.