A lost iPhone that was previously set up with your MobileMe account has a higher chance of being recovered through "Find My iPhone," the iPhone tracking tool in your MobileMe account. Available for the 3G model or newer, iPhones that are registered with the service use the GPS feature in the phone to pinpoint the phone's location. According to Apple, the iPhone must be turned on in order to track it as the tracking tool will display an "OFF" message if the iPhone is not connected to your cellular network.
Visit the MobileMe login page and enter your account name and password.
Click the cloud icon in the upper left corner of the browser, and click the "Find My iPhone" icon.
Re-enter your password and click "Continue." Click the name of your iPhone on the left pane of the browser. The browser will display a satellite map of your iPhone's coordinates if it is on.