Originally Posted by wodin
Thus iPhones were frequently “jailbroken” (unlocked) and a T-Mobile SIM was installed to get better service.
Had to respond to this since jailbreaking is not the same thing as unlocking.
Jailbreaking allows the installation of third party apps, whether they be ones freely made on the internet or sold from the Cydia app store. Apps in the Cydia store can be of amazing quality and are sometimes only there due to arbitrary Apple rules in denying apps such as allowing too much customization of iOS like putting icons where you want (gridlock) or allowing too many apps in a folder (infinifolder)!
Unlocking is completely separate and allows for the use of the phone on other carrier's service.
My previous iPhone was jailbroken for the 2 years I used it but was still locked into AT&T. I can't wait for upcoming jailbreak compatible with iOS 6.