Zach Epstein tracks down the first ‘potentially serious‘ flaw in iOS 7:
iOS 7 features several enhancements to Siri as well as a new Control Center, each offering a number of new conveniences. By default, both are also available while the user’s iPhone or iPad is locked… these conveniences come at a price: Using either the Control Center or Siri, someone who steals an iOS device or finds a lost device can quickly and easily disable Find My iPhone or Find My iPad, thus preventing a user from recovering his or her device.
It doesn’t really prevent the user from recovering the iPhone. It makes it more difficult. The user can change the setting which prevents the problem.