Nope, but I'd be up for a trip one day if anyone wants. I don't know of any geocachers in Korea, but the xkcd code for generating geohashes based of the US stock exchange closure data would work globally, so if enough of us are interested we could simply pick a sector, pick a date, and then try to meet up at the designated place, at the designated time provided the location is not unreachable without a charter or walking into a minefield and/or NorK.
The following sectors appear to be the best in the nation:
East Seoul / Gyeonggi [37/127] Good subway / Korail and bus coverage for the most part. No DMZ, NorK or ocean.
West Seoul / Inchon [37/126] Best subway coverage, but also includes ocean and the DMZ. Getting one on a small accessible island would be a good laugh.
Daejeon / Chungchong [36/127] Has a metro area right in the middle (Daejeon) with good intercity bus services. Well positioned but few trains unless you happen to land to a metro area.
Jeju [33/126] Unbelievably good location. Jeju and ALL islands accessible from Jeju fit perfectly in the sector. WOW! Unfortunately, most of us don't live there so this is off limits.
Here's the code, and there's a link to the implementation in the corner. A wiki for it exists too. If you choose to go to a geohash location,
avoid the man in the hat.
http://xkcd.com/426/Up-do: You don't need an iphone or GPS if you pick a landmark as whilst Google's coverage of Korea is justifiably rubbish, Naver Maps and Daum have AMAZINGLY detailed located map data. Basically, pick a spot you'd like to hide it, then go hide it and find the coords later via satellite map data. If you want a suggestion, I'd say a) find the Choson-era escape hatch by the former Russian Consulate and see if its possible to slip it in there behind a rock or b) hit up the abandoned Japanese military tunnels around Busan or c) Mara-do lighthouse. Koreans would probably love you if you stuck it on Dokdo, but that's just asking for strife. If you choose one of these spots and wish for me to erase this part of my post, please PM me.