My wife is manager at Best Buy in EasternMA. They will take pre-orders on every configuration of the 6 and 6 plus. pre-orders will be IN STORE ONLY from 10am to noon. They haven't currently been informed what the numbers are that they can pre-order, but even when they will, she states this is just the company's best guess. The pre-order is $50 in the form of a gift card (which you can return when you pick the phone up if you want).
So... it's entirely possible that even with a pre-order you won;t get your phone for some time. I know the 5S had extremely ridiculously limited stock.