Perhaps its because of convenience alone, but after contemplating using an armband with my iphone, I just spend $39 at the refurb store and got a shuffle. It's not nearly as cumbersome as having your iphone on your arm, and also theres no risk of damaging it with sweat, gym equipment, dropping, bumping, etc. A decent armband will cost you $25-$50 anyway.
Not quite what you were asking, but it's an idea.