#insert SD card
# mkdir /media/card/boot-image
# mkdir /media/image
#256MB image
dd if=/dev/zero of=/media/card/boot-images/loop-rootfs.bin count=512
losetup /dev/loop1 /media/card/boot-images/loop-rootfs.bin
mke2fs -j /dev/loop1
mount -t ext3 /dev/loop1 /media/image
cd /media/image
tar zxf /path/to/tarball.tgz
cd ..
umount image
sync