Now, as some of you might have heard, the rights to PunBB are no longer owned by myself and/or the other developers.
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It should also be clear that once code is released under the GPL, it remains that way forever i.e. it cannot be made closed source retrospectively.
Code that has already been released cannot be un-GPLed. However, the copyright and license on new code (future releases) is not governed by the GPL unless it is released under those terms. There is no guarantee that future releases will be released under the GPL. There are several examples of GPL projects that switched to proprietary models; Active Collab comes to mind. That is entirely within the rights of the copyright holder.
Most open source project with a certain size can't rely on pure advertisement and donation (the only exception I can find right now is Wikipedia, which is special). They all use other means, usually partnership of some kind and/or sell service around their products, to support them.
Service integration and partnerships are different concepts from selling the rights to the code / brand / etc. phpbb, wordpress, and others, for instance, offer various services and such, but the code / copyright is still owned by the developers.
Small world.