Osmocom SIMtrace is a software and hardware system for passively tracing SIM-ME communication between the SIM card and the mobile phone.
I've already got it set up and running, and played around with it a bit. Hopefully I'll have some time to do some true SIM-sniffing soon. My plan is to over time analyze all of the major cell phone providers in Norway, and let you guys known what they really are up to with their SIM-cards. All of my results will be published on my wiki.