I just phoned up and cancelled my US Airways Dividend Miles Premier World MasterCard.
Ah, that felt good.
It was weird knowing I would not be negotiating with the rep (If you've ever closed a credit card, particularly a rewards/mileage CC, you know that they'll do a lot to try to retain you).
I told the rep that I was extremely unhappy with US Airways' new policies (no more 500 mile minimum per flight), and rebuffed all efforts to get me to keep the card.
Tomorrow's the last day for Gold Preferred status. So long, and good luck, US. You're going to need it.