OMG. Next thing you know, the bad guys will use pay phones!!
you are going to see a move to restrict the anonimity aspect of these pre-paid cell phones. it has to happen, this is a gaping hole in the ability to monitor communications. not that the conversation cannot be tapped, they surely can - but you don't know who has the phone, and the same person can just switch from phone to phone, so its impossible to establish a calling pattern tied to an individual.
yes, they can use pay phones. but those at least have a geographic certainty to them - you know where the person was when making the call. and its alot harder to drive around from place to place making calls on pay phones.