When trying to open a new QDialog with .showFullscreen() on mac OS Mavericks with QT 5.2.1, it doesn't take the fullscreen.
http://qt-project.org/forums/viewthread/39346/

Basic compilable example :

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include "dialogdummy.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}

MainWindow::~MainWindow()
{
delete ui;
}

void MainWindow::on_pushButton_clicked()
{
DialogDummy dialogdummy;
dialogdummy.showFullScreen();
dialogdummy.exec();
}