I agree with medic, if not a gasket thats in the beginning stages of failure, then possibly a crack...but we wont go there. Both start off with the symptoms of slow coolant usage, white smoke on startup and rough idling on initial startup. If not addressed soon, it will progressively get worse until it finally blows fully. Then your engine will not start, and the result of coolant in the oil can lead to bearing damage. Suggest you get on it sooner than later. And as mentioned, not all that hard to fix, more of an inconvenience.
~Darin <img src="/forums/images/graemlins/baby.gif" alt="" />