Yes of course. Please read http://doc.qt.nokia.com/4.7-snapshot/thread-basics.html

As mentioned, each program has one thread when it is started. This thread is called the "main thread" (also known as the "GUI thread" in Qt applications). The Qt GUI must run in this thread. All widgets and several related classes, for example QPixmap, work in secondary threads.
To be clear: Its (by the design of Qt) not possible to work with widgets in threads. The work only in the main thread! Inform the main thread from your worker thread to show the message box.