If you live in a country with GSM based networks, (which will cover most of you) you should know how easy to get a new phone. Buy one, take the battery cover off, and slip a SIM in then you're good to go.
Well, this doesn't apply to South Korea. There are three kinds of networks here.
CDMA, WCDMA, and iDen.
CDMA and iDen has nothing much to talk about IMEI's. What is wrong with WCDMA IMEI is that all of the providers are whitelisting IMEI's. They only allow phones with IMEI's in their database to be registered on the network.
To make it easier, if you bring a WCDMA (or 3G) phone to South Korea and put a Korean SIM into your phone it would NOT work.
Now, if you roaming then your phone would work but local SIM's do not work with your phone even it has supported frequencies, etc or I can say technologicaly suitable, Korean networks would not welcome any foreign phones.
Quite stupid huh.. But believe me they've been doing this for more than 10 years. (including time when they only had CDMA networks. they whitelisted ESN's.)