The South Koreans are certainly capable of making an atomic bomb, but I doubt very much they have done this. They would have to put together an enrichment facility to process their used fuel from their commercial reactors, and thats no small thing. And if they were to try this, everybody and his brother would know about it, because to say that the South Korean government leaks like a sieve is a huge understatement.
(Personally, I'd do it on one of Kim's big "everyone is marching or flashing cards" days, without warning. Just bang over the DMZ and "Happy Pearl Harbor Day!") But, that's just me.