fetch works for me, however, it has some limitations: due to issues with
busybox, name resolution doesn't work which restricts fetch parameter to
urls with IPs in it only.
this has also the effect, that you cannot fetch from an namebased
virtualhost in your httpd configuration.
if you want to test that it works, i've copied lenny beta2
standard.squashfs to a host of mine (test it know if you want, i'll
remove it tomorrow again). best is if you boot with a image that has not
the same configuration (!=standard), so e.g. kde or something like that,
so that the image you will fetch is clearly different.
then, use this at boot prompt:
live fetch=http://213.239.197.46/standard.squashfs