UMTS (3G) prepaid card in Korea: easy to obtain? Cheap?

Hi,

does anybody know whether prepaid UMTS (3G) SIM cards are easily available in Korea (for tourists, i.e. people w/o a residential address in the country)? And what pricing could I expect for the services? Time-based or volume based? I'm primarily interested in data not voice.

Or is WLAN so ubiquitous that I can count on guest houses, motels, minbaks, restaurants etc. all to offer WLAN included?

Thanks