An Apple Store employee told me that it's possible to check out of an Apple Retail store using an app on your iPhone (is this true?).
Is there a similar app for Android, Blackberry, or any other device/platform?
To answer your question about ios devices yes can with this app, https://itunes.apple.com/us/app/id375380948?mt=8
I can't find the article that touched on using other devices but the gist of the story was if you wanted to use another device to purchase items, call the store give them your information then pick it up in the store. (sparse attempt at humor)