Quote Originally Posted by Hamish Wilson View Post
There is nothing preventing anyone building a commercial game based on id Software's released engines, you just have to keep the engine code that you build on under the GPL.
What you describe here is the LGPL. The GPL requires you to also release the code that links to the GPL'd code under a GPL compatible license. In this case this means that not only the engine source code but also the game source code must be released under a GPL compatible license.

For example, Steel Storm is a commercial game based of the DarkPlaces engine. You can not keep the engine code proprietary, but your game data can still be proprietary if you so choose
Sure the game data can be proprietary. But the makers of Steel Storm have also not released the game source code and are in direct violation of the GPL.