"Northwest is saving some preferred seat assignments (including exit, aisle and window seats) for domestic and international travelers until check-in. With Coach Choice, you can confirm these preferred seat assignments 24 hours prior to departure for a modest fee. Domestic seats range from $5 to $35."
It would have gone over better with me if they had said that people willing to sit in a middle seat were getting a discount. But there's no way to sugarcoat that they seem to be taking away the ability to choose a window or aisle seat when you purchase your ticket.