From the KT Expat Blog:
"As of January 24th, 2011, the Korean Communications Commission has enacted a new policy allowing phones purchased outside of the Republic of Korea to be used locally, without need of a
Certificate of Broadcasting and Communication Equipment (“방송통신기기인증서”, also known as a “전파인증서”). All that is needed is (free) registry of the device’s IMEI and serial number into our system.
(Previously, non-Korean phones were not usable on Korean networks without a prior costly certification through the KCC.)"
KT's Expat Blog :: Use of non-Korean Mobile Devices with KT