I haven't updated my iPhone 4 since still on 1.59 baseband, but wasn't this feature already available for iOS5? Hold finger on area you want to focus on. A few seconds, it will lock on that item.
Camera
Take advantage of the built-in cameras advanced features. New APIs let you control focus, exposure, and region of interest. You can also access and display faces with face detection APIs, and leverage hardware-enabled video stabilization.