I am planning to add a real cedar roof and siding to our house. I want to do it properly so that we do not have any moisture problems which I hear is common. Cedar is attractive and expensive. As you noted, moisture trapped under it can cause serious problems. Make sure your contractors are experienced with this.
Poor installation is the cause of most problems.
One of the best products for allowing the cedar to breathe is Cedar Breather made by Benjamin Obdyke. It creates a continuous air space between the cedar and the solid sheathing to let moisture escape quickly.