in america they lock the phones so that only att sim cars will work on the iphone in other countries like greece they sell them for extra cash and you can use any sim and not locked to one carrier like vodafone,t-mobile att, and so on.
like in the uk if you buy it from the apple store they dont lock the phone but buying from a carrier like vodafone its locked to them. so if hes giving away a free iphone 4 in the us it wont be good if its locked to att