On Fedora 9 with packagekit 0.2.5 :
running /etc/cron.daily/packagekit-background.cron from the package PackageKit-cron give this error :
Option 'update-system' not supported
but running /usr/share/PackageKit/helpers/yum/update-system.py directly works.
Fixed in 134ea622cbeba4ba3de9761a99db9dec07282018, many thanks.