Since the iPhone 3GS, you could tap on any part of the image in the Camera app to lock focus and exposure. The exposure and focus however shifts as you recompose your image.
New in iOS5, you can finally lock focus and exposure on a spot and keep it locked even as you recompose the frame. But it’s not entirely easy to figure out how.
Tap and hold on the spot for 2s until the focus box animates. The words ‘AE/AF Lock’ appears at the bottom of the screen as you let go of your finger.