On my last visit to Seoul, I rented an Android phone, Nexus One, from S Roaming. The rate was very good as I got 50% off the regular daily rate of 2,700 KRW (~$2.50 US) with the data rate of 1 KRW (~$0.001 US)/1 KB. Nice feature is that the daily data rate is capped at 10,000 KRW (~$9.23). The unlimited data plan is available at 5,000 KRW (~$4.62)/day which runs through the end of June 2011. Additionally it takes online reservation so you can reserve one before you get to Korea. S Roaming is available only at Inchon and Gimpo airports.