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According to a post in the Apple Support Communities:
There is GOOD NEWS for people with iPhones bringing them into Korea! As long as it's factory unlocked, you CAN get your iPhone to work in Korea. The best way is to go to a KT global store and first have your iPhone's IMEI registered into the Korean directory. This is NOW FREE, one foreign IMEI registration per person. Then you should be able to proceed in getting a micro sim card and catching network signal, again, assuming your iPhone is factory unlocked. No extra fees. It's just like activating a new phone in Korea, except you don't have to pay for a phone since you have your iPhone.
So I searched in the KT webpage and it's true that you can activate 1 device. There are no fees, you only have to carry your passport. If you want to register another device, you have to de-activate the first one. Of the website there are also a list of all shops