It looks like iPhone 5 has another bug, or rather feature. It seems that all it takes is to plug in an AT&T SIM into a Verizon iPhone 5 and you are up an running on the AT&T network. Apparently Apple forgot to SIM lock the Verizon iPhone 5.
It's the first time Verizon iPhones have been able to access AT&T's network without complicated hacking procedures. The feature gives Verizon customers the option to switch carriers rather than renewing an expired contract.