Those avoiding hacking their Apple TV to add functionality don't have much of an excuse as of today. Apparently, you can hack the little media device without even opening it. While technically that means your warranty is still valid, I'm sure Apple would be able to dig up some dirt on how you tinkered around with the filesystem. Details are at the usual place.
Booting from USB requires booting the Apple TV as normal then holding â€˜menuâ€™ and â€˜-â€™ keys on the remote to initiate a restore boot. From there it takes a bit of timing, Turbo explains, â€œIf I hold down menu and - from poweron until the first screen flash, I believe that it does the equivalent of cmd-opt-p-r. I only got it to boot from USB by releasing it at that point, then hitting it again just before the screen flashes on the second reset.â€