Very bad idea. Just because someone gets closer to the device, does not necessarily means that they want to perform zoom in or out of the content. Looks like Apple does lose its creativity with bad ideas running amok in the company pretending to be new innovative ideas.
A much better idea is to detect the movement of the fingers without requiring the finger to touching the surface, which will avoid the oily finger print issue. Of course, to make it viable instead of accidentally detecting unintentional movement, there should be a small distance the fingers need to be from the surface, and there should be onscreen feedback to indicate what gesture is being detected. This approach, is already partially done in Samsung Galaxy Note 2, except currently it still requires the use of a stylus.