Alright, this seems to be something WM specific then. While it worked on Windows the window stays inactive on X11. Try this:

Qt Code: 
  1. // main.cpp
  2. #include <QtGui>
  3.  
  4. class MainWindow : public QMainWindow
  5. {
  6.     Q_OBJECT
  7.  
  8.     public:
  9.         MainWindow(QWidget* parent = 0, Qt::WindowFlags flags = 0)
  10.             : QMainWindow(parent, flags)
  11.         {
  12.             setCentralWidget(new QTextEdit(this));
  13.         }
  14.  
  15.     public slots:
  16.         void activateWindow()
  17.         {
  18.             QMainWindow::activateWindow();
  19.         }
  20. };
  21.  
  22. int main(int argc, char* argv[])
  23. {
  24.     QApplication app(argc, argv);
  25.     MainWindow win(0, Qt::Tool);
  26.     win.show(); // must be called before setting Qt::WA_QuitOnClose
  27.     win.setAttribute(Qt::WA_QuitOnClose);
  28.     QTimer::singleShot(0, &win, SLOT(activateWindow()));
  29.     return app.exec();
  30. }
  31.  
  32. #include "main.moc"
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PS. Could you update your profile to indicate used platforms, please?