It’s actually pretty easy to shoot sneaky pictures of people using the iPhone. You can pretend you’re doing something else as you point the camera at your unwitting/unwilling subject, or you can just hold the iPhone up to your ear, walk past them and snap a picture using the volume button.
Now there’s a third way: the Smartphone Spy Lens, an add-on that lets you shoot sneaky shots at 90-degrees to the camera’s own axis.