I was there just a few months ago for work. Speed traps and cameras everywhere. And I mean everywhere. They even calculate your average speed on major highways and so on. If your average speed is over the posted speed limit within given a stretch, they will kindly mail you a ticket or two.
Better find a track... public roads are heavily monitored in Korea. It's the most networked and wired country in the world, and so are the roads.
There is a 'racing park' in Taebaek city (http://www.racingpark.co.kr/use/program.asp
) . I think it costs about KRW 110,000 (USD 100) to buy a pass. Never had a chance to drive there myself... but I hear it's the best track that is publicly available.
There is also the Korea International Circuit in Junnam Province, but it won't be completed until mid 2010.