You can use QuickTime on a Mac to record content playing on an Apple TV.
- Create a new Movie Recording with QuickTime (under the File menu)
- Next to the record button, click the menu to choose camera and audio sources
- Your Apple TV(s) should appear as a source in both options
- Select it, and QuickTime will present a random code to authorize your Mac to record it
- Content from your Apple TV should start streaming into QuickTime, ready for recording
Content with DRM (like iTunes movies) seems to error out, but ungracefully (for me, QuickTime just throws a ‘could not connect’ error). YouTube works fine though.