I think there are a number of things that are needed to jailbreak the atv. One of those is luck.
Recent experiance. Re-Jailbreak my atv in the longe so as i get IOS 5 on it etc, went smooth as a babies bottom, no issues, jailbroke with no problems at all. 1 hour later, same kit (cable, pc etc) go to jailbreak my 2nd atv (from the kids room) and no luck, cant get into DFU mode.
Just goes to show that there ARE some slight differences in the atv and that you do need luck on yourside.
Still havent been able to jailbreak the 2nd atv and have tried about 10 times now! :(