How to Use iPhone Locator to Locate Another iPhone
If you have lost or misplaced your iPhone, you can use Apple's "Find My iPhone" feature to pinpoint the geographic location of your missing phone. The program is available only to MobileMe subscribers, and for it to work the "Find My iPhone" service must be enabled on your iPhone and your phone must be turned on. Once your iPhone's location is found, you can choose to either lock your phone or send an alert message to it.
Go to the "Settings" application on your iPhone and select the "Mail" options.
Choose your "MobileMe" email account and then press the button next to "Find My iPhone" to enable the service on your phone.
Go to the MobileMe website (see Resources) and log in with your MobileMe username and password.
Click on the "Find My iPhone" button at the top of the page and then re-enter your MobileMe password. The service begins searching for your iPhone's location. When the GPS location is found, it is pinpointed on a Google map.
Click on the "Display a Message" button to send an alert to your lost iPhone.
Click on the "Remote Lock" button to lock your missing iPhone and prevent others from using it.