Apple Find My iPhone
This is another one of the good things about the release of iOS 5 it comes with an app called "Find My Device". How this essentially works is that if you loose a device and start to panic. Apple says calm down and first sign in to the App using another device and Find my device will show you on a map exactly where your device is at the current moment. Something to know is that find my iPhone must be enabled in the iCloud settings for you to be able to locate it with this App.
How does Find My Device work
If you loose your device you will need to download and use this app on another iOS device and then sign in with your apple ID. Once your device is located it will be displayed on a map. you can then display a message or play a sound. After which you can remotely lock your device and erase your data on it.
For it to work you would need to have iOS 3.1.3 or later
Get the Find My iPhone app now.
Source: Apple