Check out this article that talks about a security flaw in the GSM cellular phone standard. GSM cover 72% of the current cellular market in 70 markets. The GSM Association says that the flaw is known and that it will be difficult to exploit. It would require that "the attacker to transmit distinctive data over the air in order to masquerade as a GSM base station. An attacker would also have to physically stand between the caller and the base station to intercept the call."