Trolltech have recently dual-licensed QT for Windows under the GPL (as reported by scc). Previously, only the non-Windows versions were available under the GPL. Reading their FAQ on the subject, I did notice a clever licensing hack.
Q: I want to dual license my software too! That means I can use the Open Source edition for development, right?
A: No. In order to write software that will be sold or licensed commercially, in addition to open source, all of your programmers will need a commercial license from the outset of the development phase.
But how can this be? The GPL allows you to use software you’ve obtained under it in any way you choose. “The act of running the Program is not restricted.” How can Trolltech prevent you from developing your app against the GPL edition, and only buying a commercial licence when and if you want to release it?
Here’s the trick:
If you use the Open Source edition of Qt, you can only publish the software under the GPL. Trolltech’s commercial license terms do not allow you to develop any proprietary software using the Open Source edition.
The commercial licence terms presumably say something like “you can’t use this license to distribute code you originally developed against the GPL edition”. So they use the commercial licence terms to restrict the use of the GPLed version. Clever. And legal.