...tinkered a bit with GPIO and Piface Digital. It was all ok until I had installed RPi.GPIO via pip. After that the directory /sys/devices/virtual/gpio disappeared and never showed up again. I had it working earlier when I installed RPi.GPIO from tar.gz.
Yesterday I made a fresh install, and there was that directory. But again, after installing RPi.GPIO over pip it disappeared again. I have removed that package, rebooted several times, deatached the Piface Digital but no luck. Don't know, what else I can do except reimaging.