I little late to the discussion, but i used my iphone for a while before I got a garmin, mainly because I wanted heart rate and cadence.

My wife had an old rubber case for the iphone that had a place to wrap the headphones on the back, like a .25" little rubber pad.

Three tie wraps in holes bored through the rubber pad around the stem and the iphone was magically secure. It would be less secure with a plastic case, but if you can find a rubber case with anything that a tie wrap would go through, it was super easy and secure.

MotionX for me too.