Update: Easy way to get Apple TV boot.efi

Apparently, the file lives in the "firmware" DMGs that Apple offers for download. The most recent version at the time of writing is available from Apple here. You might find a more recent version listed at the bottom of the page on the AwkwardTV wiki, which also features a great collection of information about hacking with Linux on your Apple TV.

To get a copy of boot.efi (on a Mac)

Download the file, and mount it by double clicking; it should mount as /Volumes/OSBoot. On this mounted filesystem, navigate to System/Library/ and copy the boot.efi file you find here. That's all!

The previous post is still valid as a means of ssh-ing into you Apple TV, fun for tinkering anyway :-)