Nothing. EXIF data is editable. MacRumors covered this bit a couple of weeks ago and concluded that it was probably authentic, although their logic escapes me. That story links to the source story at Mobileguerilla, where the basis for authenticity are the following:What proves that that is from an iPhone?
Here you go... your choice of instant iPhone flash.The iPhone must have a flash. Maybe there's a way to add a flash if it really doesn't come with one?
Seems relevant to the photo taken with an iPhone to me.