My brother will soon be traveling to South Korea for work an he has an unlocked iPhone 4S from Sprint. According to the Apple website, the 4S will work on one of the regional carriers so what he was wondering is if there are any fees associated with using a foreign cell phone while there. From some experience I have in Europe, it's as easy as walking into a carrier shop and purchasing a SIM card, but my brother has read online that there are high fees for registering the phone with, say, SK Telecom.
So, for anyone who has experience with using a US (or European) iPhone in South Korea, what is the process?