Just for record, even if you don't need it in your case you can make the signal const. This works for me:
Qt Code: 
  1. #include <QtGui>
  2.  
  3. class test : public QObject
  4. {
  5.     Q_OBJECT
  6.  
  7. public:
  8.     void emitSignal() const
  9.     {
  10.         Q_EMIT sig("This is a test!");
  11.     }
  12.  
  13. Q_SIGNALS:
  14.     void sig(const QString&) const;
  15.  
  16. public Q_SLOTS:
  17.     void slot(const QString& str) const
  18.     {
  19.         qWarning() << Q_FUNC_INFO << str;
  20.     }
  21. };
  22.  
  23.  
  24. int main(int argc, char *argv[])
  25. {
  26.     QApplication app(argc, argv);
  27.  
  28.     test t;
  29.     QObject::connect(&t, SIGNAL(sig(const QString&)), &t, SLOT(slot(const QString&)));
  30.     t.emitSignal();
  31.  
  32.     return 0;
  33.     //    return app.exec();
  34. }
  35.  
  36. #include "main.moc"
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