Seems to have been a board-specific error -- perhaps solder that shorted something, somewhere? I pulled out a new board from my pile and populated it a component at a time and everything works as expected!
Paul__B: Could you elaborate why that is a blunder? I'm not certain what the best practice is, but I tried it with both a pull-up to +12V on a breadboard and it worked just fine. Because the arduino is pulling down, shouldn't it be fine as long as the arduino can sink the current (~1 mA?).