from all my research:
iphone 4 will work under att as 'roaming.' in order to use another sim card under the major carriers (ktf/skt/lg) the phone needs to be under the IMEI registration, which is damn near impossible unless you buy another korean phone (like an iphone) and then getting the IMEI number from that by hacking... a lot of money just to use your iphone.
my opinion: just rent a korean phone while you're there. hell, some of the phones are just as nice as the iphone. in the mean time, use the iphone4 as a mini ipad.
have fun in korea btw. it's been years since i've been there and i definitely wanna go back. so much has changed.