Find My iPhone (or iPad, or iPod touch) is now free
To use the service, you have to have an iPhone 4, an iPad or a fourth-generation iPod touch running iOS 4.2. If you do, you'll be able to locate your device on a map, lock it, display a message of your choosing on your device's display (so presumably someone who finds it may return it), play a sound, or, if all else fails, remotely wipe your data. The service works from any browser or any other iOS device that has the Find My iPhone, iPod touch and iPadd app installed.
Head over here for the complete instructions on how to setup the service.