QSystemTrayIcon in Qt 4.8.0 under Ubuntu 11.10 (32 bits) using the Unity desktop no longer shows the icon in the system tray. The minimal program attached shows the problem. The Qt example application System Tray (systray) also exhibits the problem. When I build the minimal application under MacOSX (Lion) everything works properly.

The bugs seems related to QTBUG-10992 and QTBUG-19158.
The following forum thread documents this bug: developer.qt.nokia.com/forums/viewthread/14066/

#include <QtGui>


int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QLabel win("There should be a new icon in the system tray");
    win.setMargin(20);

    QSystemTrayIcon trayIcon(QIcon("/<absolute path to file>/heart.svg"));
    trayIcon.show();

    win.show();
    return app.exec();
}

Note: Please adjust the absolute file path in the code above to comply with your system.