"Save your breath to cool your soup", the North Koreans don't really care if you trust them or not.
Just accept their thanks gracefully, for a job well done. It's a small step in the right direction, that if released in NK will help sow the seeds of doubt in the minds of their people that America is an agressor.
I actually think that North Korea is interested in normalizing relations. It simply CANNOT survive the way it's going right now and Kim Jong-il requires North Korea to survive because if that goes down, that means he gets to hang on a meathook in downtown Pyongyang with some officer with a good voice reading out the crimes he's committed against the state.