I had a few difficulties copying the fedora raspberry pi rootfs on the SD card.
thomas@thinky ~/Downloads % sudo xzcat f17arm-latest-arm-rpi-xfce-mmcblk0.img.xz > /dev/mmcblk0 zsh: permission denied: /dev/mmcblk0
The solution is to use a pipe and dd with sudo instead.
thomas@thinky ~/Downloads % sudo xzcat f17arm-latest-arm-rpi-xfce-mmcblk0.img.xz | sudo dd of=/dev/mmcblk0