Apple's site says it is for "service and diagnostic use." I think that is BS. Couldn't they service and diagnose the thing through an ethernet or 802.11? Wouldn't the USB port on the AppleTV be a B-type or a USB mini type plug if it were meant to be used as a peripheral? I've never seen a USB cable with a A-type plug on both ends. I'm sure they exist, but it seems counterintuitive to me, unless.... They're feeding us a load of crap.
I think they have peripherals out there waiting for introduction before they release them. Just like MS releasing the HD-DVD player separately, they're gonna do the same thing by allowing us to add a HD-DVD or BlueRay player if we want without the added expense if we choose not to.