http://www.btmon.com/torrent/gsm%20exploit

http://online.wsj.com/article/SB10001424052748703510304574626451948722542.html?mod=WSJ_hpp_sections_news

A German hacker claims to have cracked the encryption that protects most cellphone calls, potentially paving the way for others to eavesdrop on conversations.

The claim, if true, could pose a threat to many wireless carriers who have used essentially the same security on their networks for years.

The hacker, Karsten Nohl, says he and a small team broke the encryption code for networks on GSM technology, which makes up more than 80% of the world's cellular networks, to show how vulnerable they are and to make carriers more serious about security. He is expected to demonstrate Wednesday.

I haven't tried it, but I'm guessing you have to gather a few hundred thousand packets from the cell phone, and then run the decryption program, which would probably take awhile, so it's probably pretty silly and pointless, unless you're a big secret spy or something.
Most calls would probably be over before you could decrypt the packets. I don't really know , but that's what I would guess.