South Korea and China have a hotline which, as Strategypage reports, is intended to reduce chances of incidents at sea and in the air:
China and South Korea have set up a military hotline, to avoid accidental clashes between their sea and air forces. At each end, military personnel are on duty 24/7, prepared to quickly defuse any encounters.
I can only assume that clashes between land forces isn't mentioned because the only place their land forces might want to avoid accidental clashes is inside North Korea.
Which doesn't mean that there is no need to avoid clashes between Chinese forces going south and South Korean forces heading north should North Korea experience state collapse instead of just regime collapse.
But it does mean that nobody considers it wise to mention that possibility while the excitable types in Pyongyang still exercise control over their still-deadly but rusting military capabilities.
I wonder where the line of division has been set? I assume the Chinese want the line far enough south to encompass the nuclear facilities lest Western experts learn too much about Chinese support for Kim Jong-Il's nuclear project.