Apple has acquired a patent for their 'Slide to Power Off' function. It's the prompt, much like the unlock screen, that pops up when you hold the sleep button on your iDevice for a few seconds.
Here's a description of the patent:
In some embodiments of the invention, a graphical user interface in an electronic device with a touch screen display and power switch includes one or more user-interface objects to assist a user in powering-off the device while avoiding unintended power-off events due to inadvertent contact with the power switch or the touch screen display.
It's basically what it sounds like, it's designed so that devices aren't powered off by mistake (either by hitting a power switch or an accidental touchscreen input) when the user has the device in their pocket or bag.