Network programming and the main thread

[jacek]

The app.exec() or another event-loop?

That thread's event loop (QThread::exec()).

So basicly the problem I described is also valid for QT4 ?

On the contrary, there is no problem in Qt4 (at least when it comes to networking and threads). If you flood your application with thousands of UDP messages it will become unresponsive, just as any other program.