Until recently, European travelers to South Korea could not use their mobile phones due to incompatible wireless standards. Recently, however, KTF and SK Telecom have chosen to change standards and are now deploying UMTS/HSDPA networks (see here and here).
To use these networks with "European" UMTS phones, two other things need to be in place. First, these networks must be deployed in the 2.1 GHz frequency band, which is the band supported by European UMTS phones. Second, the home operator must have a roaming agreement with at least one of the operators that are deploying HSDPA networks.
Looks like all three things came together for a colleague of mine who traveled to South Korea recently. He's a subscriber with T-Mobile Germany and was able to use KTF's UMTS/HSDPA network without any trouble with his Nokia E-60 3G phone. Funnily enough, T-Mobile lists KTF still as CDMA network in their roaming data base.
Similarly in Japan, foreign visitors from Europe can now also use their 3G phones since NTT-DoCoMo and Softbank (former Vodafone K.K.) both operate UMTS/HSDPA networks in the 2.1 GHz band. Not many places on earth now anymore with a wireless network in range where a GSM/UMTS phone can not be used. For a world wide overview of GSM/UMTS coverage, take a look here.