What makes you think Standard US Cellular Network (AT&T, T-Mobile, etc) GSM traffic isn't encrypted? It is encrypted, just that that at least some of the older ciphers used have been broken. Gr-GSM (which is mentioned used in the link) has some basic implementation to decrypt traffic built into it. I've only skimmed it over, so I'm no expert on the project, but I wouldn't go around sniffing a cellular network's traffic without some careful consideration of what the tools you are using are doing.

http://en.wikipedia.org/wiki/A5/1

https://github.com/ptrkrysik/gr-gsm/blob/a33f3d82ed565719645...