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.
Monday, January 26, 2009
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 :-)