My T-mobile phone worked fine in Korea, AT&T should work just fine. CCs worked in most places, but don't expect to be able to understand what your receipts says which is usually a good thing hehe.

You only really need cash for street food and the odd taxi/restaurant that don't take it. But Korea is pretty cheap, especially taxi. Just bring some extra cash and enjoy!

Nothing like loading up your wallet with a couple of hundred thousand (won..) and hitting town