Pinch-to-zoom with one hand is extremely awkward, unreliable, and inaccurate. It's also a great way to drop your phone when you're standing on a crowded bus.
The root of the confusion here is that iphones are a lot smaller than most android phones. I'm holding a blackberry bold right now, and pinch with two fingers while holding the phone with three works just fine and is plenty secure.
If you're right handed: bottom right corner in the fold of your palm, supporting the back with your ring and middle fingers, and support the bottom edge with your pinky. Works fine as long as you're not gesticulating aggressively with the same hand.
