You're probably thinking of Chris Paget's Defcon 18 talk demonstrating an OpenBTS/USRP/Asterix based 2G IMSI catcher. Also, it's not entirely illegal to operat

You're probably thinking of Chris Paget's Defcon 18 talk demonstrating an OpenBTS/USRP/Asterix based 2G IMSI catcher. Also, it's not entirely illegal to operate on some cellular frequencies which overlap with ham bands, if you're a licensed ham radio operator, which Chris apparently was (KJ6GCG). He also had to disable encryption (illegal on ham bands), use extremely low power and a highly directional antenna (to ensure he didn't intercept anyone outside of the room), and ensure everyone inside the room was aware of the demonstration (IIRC there were signs outside the room). He also destroyed the USB stick the base station was writing to. Even then it was definitely a legal gray area...