One interesting, but unlikely possibility is an originate-only phone which pays for calls with cash (e.g. Digicash, or a prepaid phone activation card). Since it can't receive calls, it doesn't need an identity. What would come out of the system is, "Someone in cell X called telephone number Y."
There's already a way to do this: cloning. This is not always done just to avoid paying for service. Certain cloners are entirely able and willing to pay for cellular service, but they demand anonymity.
Somebody should point out to the carriers that they could get rid of much of the incentive to clone phones if they simply offered a legit way to remain anonymous.
Phil
This technology already exists in Britain (I don't know about any other countries), where you can buy a mobile without any subscription information off the shelf. To use the mobile, you go and purchase a 'token' which allows you to use the mobile on a pay-per-call basis much the same as a public phone. I don't know the method of token implementation. The police have started kicking up a fuss over this technology as they claim it hinders their investigation into criminal activity, because if they trace a cellular phone which turns out to be one of this type then they can't pull the info on the customer to go round knocking on doors. Geraint