So you've made a great iPad app, and you want to tell your friends about it. However, you can't remember which of your friends have iPhones, iPads, or Android devices. It's annoying to send your friend a link to an app which they can't run!
It's an array of objects, each with a "os" (iOS or Android) and optional "hardware" (iPhone or iPad, for now).
Check it out: http://apps.facebook.com/friendswithdevices/