Qt supports different native styles, Qt looks - and behaves - native in Windows, OS X, GNOME, KDE, ... Try QtCreator for example, or VLC, or VirtualBox, or any other application made in Qt.

Check out their whitepaper "Qt Cross-Platform Application Framework": http://qt.nokia.com/files/pdf/qt-4.4-whitepaper

I wrote a small mail notifier a while ago, it displayed fine on Linux with minimal changes. I don't have any other examples handy, though, sorry...