Re Riverside forums: PyQt isn't Qt and isn't even the official Python bindings for Qt, but a third-party product. Only a small part of Qt is GPL, the vast majority is LGPL (always with commercial as the alternative). (Indeed, given that PyQt is GPL + commercial third-party, it seems to show that dual-licensing components that link to Qt is indeed possible)

Only if you're unwilling to be using GPL in the first place, which is clearly the desired effect. (If somewhat unfair for small companies, that have good reasons for not using a libre license and can't afford a multi-licensing expert.)