If it's an officially unlocked and jailbroken (note that these are two separate processes, and unlocking is the important one to you in this instance) iPhone 4 you'll have no problems. Unless you count Koreans being unsure that unlocking is possible (as it's not possible everywhere) and/or being worried about explaining in broken or no English about the unlocking process to be a problem. Either way, you should just be able to pop an iPhone 4 sim from SK or KT into the phone, plug it into anyone's itunes (as long as itunes can see the internet) and have it work. Just be sure it stays unlocked: ie. IF your unlock is based on the jailbreak, for instance, instead of it coming pre-unlocked, then you'd better not do anything to unjailbreak it, or you'll be holding an expensive ipod touch.