There are a few things wrong here.
1- the iPhone does not have the ability to use the headphones as an antenna.
2- radiation is all around you. It is a myth that having a cell phone near your head increases radiation by any effective amount is false. Maybe about .5 watt or less but you get hit with more just living in a city - think fm and am radio, wifi, cell phones etc.
3. We know exactly how much radiation comes from the iPhone. It's called the FCC.
Yes I know. But we don't know how much radiation travels haphazardly up the earphones, due to them acting as a physical conductor being attached to the metal iPhone. It wouldn't be traveling up through the audio wire of course.. just hitching a ride on the outside of the cable, very much like what happens with the wire telephone game.