Thanks to a tip from @blackthorne I have the gr-qtgui component of GNU Radio working on Mac OS X Snow Leopard as well. As reported earlier, previous installations worked well except the Qt GUI part which gave errors like the one reported in my email.
Since I already had py26-pyqt4 and python_select installed all I had to do was to execute
sudo python_select python26
sudo port install py26-scipy
then reboot. I could then execute the pyqt_example.py and qt_digital.py examples located in the GNU Radio source tree under gr-qtgui/src/python/ – see the screen shots below.