USB port not functional by default
According to Apple, the USB connector on the Apple TV is reserved for Apple support only. Indeed, it does not work on the stock Apple TV OS. However, given that Command-S will boot it into single user mode it seems unlikely that they've disabled the port via EFI.
How to enable USB
There is no known solution at this moment to use USB devices with Apple TV OS, but it will be posted here once it becomes available. (Some people have tried swapping IOUSB files, but that didn't work.) However, it is possible to boot from an externally attached USB drive (see below).
Command-S boots single user mode
However, the USB port is fully functional hardware-wise:
Plug a keyboard in and press Command-S during startup will start up in Single-User mode.
Boot from USB drive
Ozy used a cloned internal drive where he changed the boot picture in order to distinguish it from the internal drive. Since the Apple TV OS disables USB during the boot process, it doesn't fully boot. But hey, it's a start, isn't it? Next we'll make a Darwin mini USB drive image...