While the North Koreans have a large military with fairly modern equipment, the South Koreans also have a modern military with up-to-date equipment. I really doubt the South will be taken by surprise as they were in 1950. I don't think the US will be much of a factor in any conflict with the North, at least initially, and we don't have any significant ground forces anywhere close for reinforcement. However, I think the South can probably take care of the North in any conventional engagement. Where I think we need to we wary of North Korea is in their support of other world wide terrorist organizations.
Some years back, when I was still in the military, my friends who were stationed in Korea said it appeared the US didn't have troops there to protect the South from the North -- our guys were there to protect the North from the South... Apparently the ROK military was ready and willing to go sort the Norks hash once and for all. dunno if it is still that way tho.
Altho I'm sure our present CinC and government would do everything it could to kick the legs out from under the South if war did break out.