I was in Seoul last summer and drank tap water and didn't get sick. YMMV.

A couple of guide books said not, but that seemed really strange to me.

There is an interesting scene in Slumdog Millionaire. One character (Jamal I think) is working in a restaurant. He picks up an empty plastic water bottle and fills it from the tap and recaps it and then runs a thin line of glue around the cap.