Just to let you know: I recently tried to re-jailbreak my ATV2 after some stuff had gone badly wrong with the media player.
To my surprise the ATVflash (black) installer didn’t let me install - it gave the “cannot proceed” warning, claiming that I either have an outdated version or no internet connection.
I tried this from two different machines that have lion installed with the same result.
I was lucky that I still have one “mac” that still runs snow leopard. From there, the installation worked without a hitch.
This might be just coincidental, but it might also be a Lion compatibility issue.