But you have to make sure to start Find My iPhone or it doesn't work.
iOS 7: Activation Lock secures your device in case of theft (Updated) | TUAW - The Unofficial Apple Weblog
Activation Lock improves Find My iPhone by preventing a person from disabling Find My iPhone without knowing both the Apple ID and the account password of the original owner. The person who has the iPhone also can't restore it or reactivate it without these account credentials. They can only stare at the phone number and a custom message that asks the person with the phone to return it to its rightful owner.
To use Activation Lock, you must first turn on Find My iPhone in the Settings. Open the Settings app, tap iCloud and then Find My iPhone. Make sure the slider is set to the on position (it will be green).
That's all there is to activate it. The magic happens when the iPhone is lost or stolen.