How would such a feature know when not to zoom as you approach the screen to touch it for other functions? I guess a slight delay may work, but I'm not sure if the calibration could be set fine enough for a feature like that to not constantly annoy someone by zooming when not desired.
That's pretty trivial I think. It would function when web browsing which only works when in the browser. You don't need zoom when you're navigating the icons of the main GUI "desktop". Double tapping to zoom in when web browsing is a PITA given the zoom is preset and not variable. Pinching to variable zoom in/out is a pain too and requires two hands...one to hold the phone and one to pinch. A proximity sensor for fingers would allow zooming and holding of the phone with one hand.