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Unity, even has adopted it for their 2d interface and ubuntu TV. By coding it in Qt you make it native in KDE, and look native in GTK environments since Qt has a gtk theme engine and a clearlooks theme.
You can check the following wiki for some more information
Furthermore you will have the advantage of a single codebase that can work on all of the platforms, which brings home the Java montra code once run everywhere. Unlike java the programs won't have a shitty theme but would look / act more or less native. Plus you gain access to a 2d canvas and a declarite API with Qtquick.
Well I'm actually planning to not use a single toolkit but instead have one for each environment. So there's no reason for me to go for both a Qt and a GTK interface. There will perhaps be even more since I think a Unity interface will have to differ from an LXDE interface.
I am going for the feeling that Stoffi should feel like it's part of the DE and not a third party application.