Requires iOS 4.x
Not compatible with iOS 5

You must reboot your device manually after installation. Power-down or reboot when it's safe for you to do so, keeping in mind if you have a 'tethered jailbreak'

Track and recover your lost iPhone with a single SMS.

Get the location instantly just by sending a silent text message to the phone. Features:

• On SIM change, a silent SMS is sent to predefined numbers containing the number, ICCID, and IMSI of the new SIM
• Send special commands via SMS to retrieve the location of the phone, to execute system commands etc
• Automatic GPS activation, if it's switched off
• All SMSs received and sent are silent and hidden from inbox
• You can even track your kids (assuming this is legal in your country)

See More Info below for a Screenshot