Comment 10 for bug 881292

I'm working around this temporarily by just using --image_file. If you use --image-file though, it complains. I've tried this on twi different machines:
$ sudo linaro-media-create --hwpack-force-yes --dev panda --hwpack hwpack_linaro-lt-panda_20111019-0_armel_supported.tar.gz --binary nano-n-tar-20111019-0.tar.gz --image-file foo.img
usage: linaro-media-create [-h] [-v] (--mmc DEVICE | --image_file DEVICE)
                           --dev
                           {overo,vexpress,mx53loco,snowball_emmc,panda,ux500,igep,mx51evk,smdkv310,snowball_sd,beagle,efikamx,origen,efikasb}
                           [--rootfs {ext2,ext3,ext4,btrfs}]
                           [--rfs_label RFS_LABEL] [--boot_label BOOT_LABEL]
                           [--swap_file SWAP_FILE] [--live | --live-256m]
                           [--console CONSOLES] --hwpack HWPACKS
                           [--hwpack-sig HWPACKSIGS] [--hwpack-force-yes]
                           [--image_size IMAGE_SIZE] --binary BINARY
                           [--binary-sig BINARYSIG] [--no-rootfs]
                           [--no-bootfs] [--no-part] [--align-boot-part]
                           [--nocheck-mmc]
linaro-media-create: error: one of the arguments --mmc --image_file is required