Mountable filesystem image

["Brendan Simon (eTRIX)" <brendan.simon@etrix.com.au>]

This is a bit off topic, but I want to use something like this for
booting emdebian linux systems :)

Anyone know of a filesystem image format that is mountable and writable ??

I was thinking of something along the lines of the qcow2 format for
QEMU, but something that is mountable by the linux kernel.

Essentially I want an ext2/ext3 rootfs (and possibly others) filesystem
image that lives on a compact flash card that the linux can mount and
use inplace as filesystem.  i.e. all modifications would get written
back to the filesystem image.

I do not want to format and partition the CF card into ext2/ext3
partitions.  I want to keep the one FAT partition and have multiple
linux filesystem images as files on the FAT partition.

Any ideas ??
Cheers, Brendan.