Why not let the developer decide, if he uses the KDE theme for his software? He can decide if he wants to release his software under the GPL, then he has no problems at all. If he wants to write a commercial app, he either has to buy a license from Trolltech or he has to set aside using KDE theme for his app.
Maybe the build linking KDE libs (and therefore using KDE theme) can be controlled by setting another build flag.
Matthias
Actually, build do not even has to link to KDE libs. You can use dynamic loading...