IMO the guarantee is that they paid far more money just to get Qt than they ever did spend on GTK coders and development. By owning ex-Trolltech, now they have a far more vested interest of keeping it (and applications based on it) alive and kicking. Also, do not underestimate what owning actual upstream (!) means. In fact, I'd bet serious money that even considering the demise of Qtopia/Qt Embedded, the next Qt versions will somehow find new love for mobile devices