So, if you've got an old remote control from a device you no longer use, you can repurpose it for the Apple TV. Or if you have a universal remote, you can configure it for any unused code and have the Apple TV work with that.
I wish more devices supported this. Especially for those occasions when you have two identical devices in the same room (like at work, where our video-conference system has two identical big-screen TVs). This way, you can configure them to use different sets of codes.
I'm now wondering if I can do the same thing on my Mac mini. It has an IR sensor. If I can use a spare remote to make it do things (like run the DVD player app), that would be awesome. ... It turns out that there's an app for that at least for a small subset of commercially available remote controls.