The Unlocking Process

• Several means of unlocking an iPhone --- none of which are authorized by AT&T or Apple --- exist. As is the case with other mobile devices, you can take your iPhone to a mobile phone or computer technician and have him unlock it for you in exchange for a small fee. Several third-party software programs, or "hacks," also exist to aid you in unlocking your iPhone, although do-it-yourself methods may not always be available or up to date.
  
  

GSM Technology

• AT&T iPhones function using a mobile technology known as "Global System for Mobile Communications," or GSM, which uses removable "SIM" card to link devices with carriers. If you've unlocked your iPhone, it's possible to change its carrier by inserting another carrier's SIM card into it. As AT&T is the only U.S. carrier which officially supports the GSM iPhone as of March 2011, not all features and capabilities are guaranteed to work with other carriers.
  
  

Downsides of Unlocking

• In addition to being frowned upon --- although not illegal --- by both AT&T and Apple, unlocking your iPhone presents several potential problems. First and foremost, you can't update the iPhone's operating system automatically when you sync it to iTunes, or else the unlock will reverse and re-tie your phone to AT&T. Furthermore, unlocking your iPhone voids your Apple warranty, so if you experience a problem with your iPhone after unlocking, even if your device is new, you'll need to pay out-of-pocket to fix it.
  
  

Verizon iPhone

• Like the rest of the carrier's devices, Verizon iPhones use "Code Division Multiple Acces" --- more commonly called CDMA --- technology. Since Verizon iPhones don't accommodate SIM cards, manipulating them to be used with other carriers involves re-programming them, a process called "flashing," which needs to be performed to different specifications for different carriers. As of March 2011, however, the process is only theoretically possible, as no other CDMA carrier networks support the iPhone.