In order to use your Korean-purchased iPhone abroad, there are a few things to know.
1. Currently iPhone 4S is being sold unlocked, so you should be able to slip any other operator's microSIM card.
2. iPhone 4 and 3GS have always ben sold carrier-locked; however by law the carrier MUST unlock the device at customer's request. This means that if you fax or email them a return airline ticket they will unlock it digitally. (You will also have to send them your IMEI number, which is located on your iPhone in 'Settings, General, About.') You will have to plug in to iTunes a few days later, where it will tell you you have updated carrier settings to apply. Do that and you're unlocked. Forever. The carriers don't have the ability to re-lock it; once it's done, it's done.
Just a note too that even if your iPhone contract has expired, you still have to request an unlock; however you don't need to show a return ticket in this case.
If you still have time left on your contract you can buy out the remainder (expensive), after which they will also unlock it (but make sure to expressly request this).
If you need more info you can find it below.
KT:
www.expatblog.kt.comSK: sorry... you're on your own!
This information was provided at the request of the site operator. It is, to my knowledge, current and accurate. I hope you find it helpful.