I'll write some info about prepaid SIM card of South Korea.
1. kt olleh SIMple Charge
kt Expat Blog :: Using your device has never been SIMpler?olleh SIMple!
* There are "Prepaid" packages of kt, but olleh SIMple is cheaper and also provides longer validity.
* Data plans exists:
kt Expat Blog :: Prepaid Rate Plans (Prepaid Data Plus package)
* There are olleh SIMple data packages and its price plan is the same as Prepaid Data Plus package.
* kt offers unlimited olleh WiFi access to data plan users, except for 100MB plan.
* There is no international roaming.
2. SK 7mobile
SK telink 7 mobile (click "English" on the top-right)
* MVNO service which provides cheap international call rates.
* Prepaid data: 0.01 KRW per 0.5KB (20 KRW/MB)
* WiFi: 110 KRW/day (access to T wifi zone;
T wifi (Korean page))
* There is no international roaming.