According to https://bugs.freedesktop.org/show_bug.cgi?id=857 a call to send_with_reply_and_block is not thread-safe;
This frequently causes lockups, crashes and result corruption as in the following testcase (code by Sebastian Trüg)

class MyThread : public QThread
{
public:
    void run() {
        qDebug() << QDBusConnection::sessionBus().interface()->isServiceRegistered( "org.kde.NepomukStorage" );
    }
};

int main(int argc, char *argv[])
{
   QApplication a( argc, argv );

   MyThread t1, t2;
   t1.start();
   t2.start();
   return a.exec();
}

Sometimes the two threads give different answers, sometimes there is a timeout (~25secs) between the first result and the second

Mutexing the call in QDBusIntegrator.cpp solves the last issue; however, timeouts are still occasionally present.