iPhone 5 comes in multiple versions

Unlike previous iPhones, which had all the wireless bands in a single device, the new iPhone 5 has two distinct models, with actually three different sets of wireless network bands supported.

The numbers of these two models are A1428 and A1429. The 1428 model is a GSM/UMTS device with LTE support in bands 4 and 17. The 1429 model supports CDMA with EV-DO Rev. A and Rev. B, GSM/UMTS, and LTE support in bands 1, 3, 5, 13, and 25. The 1429 model is listed again, as a GSM/UMTS model with LTE support in bands 1, 3, and 5.

Now, that may cause a problem in switching between carriers while using the same device, in case you want to continue using LTE for data transfer. It also means that those willing to switch from a GSM carrier to a CDMA carrier with their iPhone will not be able to do so at all.  Therefore, when picking up your new iPhone, pay attention to the particular number, to make sure it is compatible with your intended carrier.