If you build an image with linaro-media-create and ask it to write to file with the --image_file option, the resulting SD card image is setup with a boot.cmd which does not try to load the uInitrd. The effect is that when you dd the image to an SD card[*] then a Beagle XM board will boot the kernel but the kernel will fail to find its root filesystem.
This appears to be deliberate in that there's an explicit conditional in prepare_sources() which writes different contents to binary/boot.cmd if IMAGE_FILE is set, but there's no rationale explaining why this is done.
I think that the script should write binary-identical data to either the file (if --image_file) or the sd card device (if --mmc); if there's a use case where you might want a different boot command then this ought to be controlled by an independent option.
[*] I can't just get the script to write to the card because the script doesn't produce working images if run under karmic and doesn't work with --mmc if run in a chroot.
i think this is matt's domain; assigning to him