Apple's just-announced iPhone 5 will be available in two separate models when it hits shelves on September 21st. According to the official specs page from Apple, there will be separate CDMA and GSM versions of the phone, a break from the iPhone 4S which packed both wireless protocols into the same chassis.
The A1428 GSM model and A1429 CDMA model both support LTE, HSPA+, and DC-HSPA, but they do so on different bands for the various carriers that the iPhone 5 will be available on. Apple also lists a second A1429 model with support for more GSM bands than the A1428 version. Based on the specs provided by Apple, the A1428 will be carried by AT&T and Canadian operators (with support for LTE on bands 4 and 17), while the A1429 will be available from Sprint and Verizon in the US and KDDI in Japan (with LTE on bands 1, 3, 5, 13, and 25). The GSM A1429 supports LTE on bands 1, 3, and 5 and will likely be offered in global markets such as the UK, Germany, Australia, Korea, and Japan.
The last time Apple had to split the iPhone into two separate models was when the CDMA version of the iPhone 4 was originally announced in February, 2011. As mentioned, the iPhone 4S used a baseband chip that was able to support both CDMA and GSM radios in one unit. The separation for the iPhone 5 doesn't come as a huge surprise, though, since Apple had to release separate versions of the new iPad with LTE support for AT&T and Verizon.
If there is one thing that we can take away from this, it's that it will not be easy to transfer an iPhone 5 from one carrier to another. The A1428 model for AT&T will never work on Verizon or Sprint's networks, and the A1429 won't be compatible with AT&T's. Additionally, should you get an AT&T model, it likely won't work with LTE networks in other parts of the world.