atv does have a USB port which has led to a lot of confusion. It is a USB client not a host. The port is there so you can plug it into a computer to restore the software (or jailbreak it). Your idea is interesting in theory but I can't thnk of any way to make it work. You would need to use a crossover cable but I am not sure if the atv supports that type of network connection. However if you are going to use Ethernet anyways, why not just plug the NAS into a router and attach it that way? You would essentially accomplish the same thing without doing anything unsupported.