The guys from AwkwardTV.org have successfully booted Apple's recently-released Apple TV via USB. They also uploaded a video of the external USB bootstrap, and here's what Ozzy from AwkwardTV.org has to say about it:

...our first successful external USB bootstrap from a recovery image on the Apple TV. This will allow us to begin development of externally booting the Apple TV without having to crack open the case and modifying any of the contents at all.

In order to prove that the Apple TV is actually booting off of the hard disk attached to it via USB, Ozzy from AwkwardTV.org "modified the bootlogo png file on the recovery partition on the external disk." Check out the vid below.