Security Research Labs on srlabs.de:
SRLabs researchers investigated the SIM hacking possibilities from two perspectives: Checking how many SIMs are vulnerable, and monitoring how many are actively being exploited.
Overall it appears vulnerabilities concerns are overblown, but they do exists.
One of the findings was surprising:
Most of the messages targeted users in Latin and South America
This analogy really helped me understand that SIM cards are not simply pieces of plastic:
SIM cards are small computers inside your mobile phone. Besides their main role of authenticating you to the network, they run Java applications and can instruct your mobile phone to do various things […]
Good short and sweet overview.