I am continually amazed that Osc and other carts have not implemented this basic change to the cart, you think everyone would be screaming about it. when you make notes to your order and uncheck the NOTIFY CUSTOMER, it sends them the message anyway....funny.
In order to make private notes, you actually need to modify the code account_history_info.php file in standard osc, but I am having difficulty tring to figure out what to modify in Zen.
My vote as well to be a pressing feature, its a two second fix to implement it into the base code of Zen. I tested all carts out there (for years) and Zen would be the first to implement this necessary change. Customers should not receive an email when you uncheck Notify customer, that is just wrong. I cannot believe it has not been fixed at the core level years ago.