The the Norks haven't flown in combat in how many years? Experience counts for a lot. And this time, the ROK Army is a force to be reckoned with.
And I would not be surprised if South Korea had an atomic weapon, or three. Just because they are smart enough not to put an ad in the International Herald Tribune doesn't mean they can't. They've got FOUR reactors, and a high tech industrial base.
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.