I see the press having a field day with the AT&T purchase of T-Mobile US, citing concerns about anti-trust laws. But the real anti-trust violation is that GSM networks, designed from the ground up to deliver independence of network provider, in the US are locked to only connect you to the signal from the network you have a contract with.
GSM was designed to give any GSM compatible phone the best cell phone reception possible anywhere by finding any (open) GSM network and connecting to the strongest one available using roaming agreements with the host network provider. Locked SIM cards (those little chip cards inside the phone) prevent its users (us the people) from having the best and most seamless cell phone reception possible and prevent GSM network providers from competing on a level playing field. Locked SIMs create walled gardens and develop monopolies, antitrust laws are supposed to prevent.
So, not the purchase of AT&T buying T-Mobile violates anti-trust, but the artificial locking and limiting the capabilities of GSM did way before. The Federal Communications Commission should have disallowed SIM locking because SIM locking violates free-market principles and significantly reduces the service level to consumers.
Twenty years ago(!) I could roam Europe and receive a GSM signal anywhere, not so in the US because locking of SIMs prevents any small new GSM network provider from achieving network relevance that could make up for the cost, and thus would prevent new network entrants from even trying. Hence locking of SIMs is the real antitrust crime, not one walled garden buying the other.
Update: Since this article was written in 2011, you can now unlock your phone from the AT&T network if you own your phone outright or have paid it off. Not solving the unfree issues described above nor the ignorance of the role of government, but a get out of jail into a halfway-house kind of solution for customers at some point.