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.
I have now tried the USRP on Mac OS X using the MacPorts installation and I am happy to report that it works very well. Indeed, the USRP is really plug and play on OS X and there is no need to configure udev or anything like it is the case on e.g. Ubuntu Linux. The video below shows the FM receiver listening on the local APRS frequency.
Few days ago Michael Dickens announced that the GNU Radio packages on MacPorts have now been updated to 3.3.0 and so I took this opportunity to try it. I didn’t have MacPorts installed so I had to start from scratch; however, installation was quite straightforward because the dependencies are resolved automatically. All I had to do was to download the latest MacPorts dmg and then execute the command
sudo port install gnuradio
then let it run overnight. It takes a long time since MacPorts builds everything from source. On my iMac it took more than 6 hours. Note that gnuradio is a meta packages that depends on all GNU Radio components.