Yes, this has been brought up before. No, phpBB did not sell their code. What is really interesting is that IPS (or whatever they are called nowadays) developed their system after phpBB, and took far less time doing it. Make of that whatever you want. I'll leave this open for now unless a moderator says otherwise. Please do not turn this into a flamewar.
Again it seems IPB is not under GPL although it is free. It is similar to another very popular php product "4images" which is also very similar to phpbb. But it is copyrighted.
From another point of view, similar to other GPL licensed packages, if another php developer take phpbb code and enhance it to redevelop another product (not necessary another forum package), should it be encouraged or not? It may save time in development and focus on the new functionalities.