Privacy of 3.5 Billion Cellphone Users Compromised – GSM Code is Broken

See here: http://www.nytimes.com/2009/12/29/technology/29hack.html

"The encryption key itself does not enable surveillance of mobile calls, which must still be overheard and identified from the digital stream of thousands of calls transmitted through a single cellphone station.

The undertaking is complex because a digital call typically hops among up to 60 different broadcast frequencies during a single conversation, as the mobile network operator maximizes the use of its available bandwidth."