I'll tell you why a 3 year-old would have an iPhone...
...because his Dad lets him play simple (and not-so-simple) games on it.
My son was 3 when I got my iPhone last summer, and he was immediately fascinated by it. As a result, I would download suitable games (like pair matching, boxes, Labyrinth) or applications (drawing, doodling, flick book etc) and let him play under supervision. He's gradually progressed to more complex games so that he now knows more about chess (Deep Green) or projectile physics (Cannon Game) than most 4 year-olds probably have a right to.
He also now expects everything to have a multitouch interface...
Anyway, he's now responsible enough for me to allow him to use the phone without me looking over his shoulder all the time (although I'm never out of the room) but occasionally he does get lost, usually by accidentally pressing on a banner in an ad-supported app. So far, he always tells me when it's happened and comes to me to fix it, but there's always the chance he could end up seeing something inappropriate.
Parental controls would mean he could safely play at times other than when I am free to supervise him, which is not that often.