On OSX (at least on 10.5) it appears QFileDialog::getExistingDirectory does not observe the default directory passed to the function. For example:

QString defaultLocation = "/tmp";
defaultLocation = QFileDialog::getExistingDirectory(NULL, "Choose Location", defaultLocation, QFileDialog::ShowDirsOnly );

does not default to the /tmp directory on various machines, where it instead defaults to the Desktop directory. The other QFileDialog static functions handle default filenames just fine.