I guess you can take any phone to the KCC (equal to the FCC in the states), pay 300,000 won ($280 roughly) and they will test your phone. If it works with their networks, they will give you a certificate that you can take to the cell provider and they will set you up. Korea is strict in allowing phones. They register every IMI number (or whatever that number is) with the government.

Also, if this helps, the Iphone works here.... It was also approved by the KCC. Not sure if the Iphone and N900 operate on the same frequencies. I'll check (i just thought about that).

I think they use: 1750mhz - 1770mhz to send, 1840-1860mhz to receive. And their KT service uses 2.15ghz-2.17ghz HSDPA

Hope this helps...