I like this feature with the iPhone. You can register all of your iPhones, iPads, iTouches, iWhateverthehellitis on an account and you can track on a map or if you cannot locate it, you can send an audible alert to the device even if it's on vibrate to play if you have difficulty finding it. Additionally, if the device is lost, you can do a remote wipe to keep your stored information from being misused.
This was available previously through MobileMe at a cost, but is now free through iCloud.
edit: Don't know that this feature works if the phone is off though. She's a teen though, so it's not like that would ever happen.
