[technofiend]

Zipcar has RFID-based card access now. Sounds like they just need to cache an auth token locally when there is cell service saying how long the car may be used for. Although there's still an edge case that parking out of cell service range when the token expires will disable the car even if you extend the contract since there's no way to refresh the token.

Perhaps they could create an RFID token in their app and let the RFID-capable phones bear that to the car since it's much easier to move a phone than a car, not to mention potentially get wifi service where there is no cell service.