SK Telecom in S. Korea now allows GSM roamers on their GSM network. You use your GSM SIM card in a CDMA phone (that accepts SIM cards) see:

http://www.sktelecom.co.kr/english/p...ing/index.html

I used this with an Orange UK account with no problems a month ago. My UK number worked fine. If anything it was less problematic than other asian countries. Calls from the UK to S. Korea terminated in a few seconds (as opposed to 20-30 seconds sometimes to other countries)