Preheat the oven to 400 degrees F (200 degrees C). In a cup or small bowl, stir together the milk and vinegar. Let stand 10 minutes, or until curdled.
In a medium bowl, stir together the flour, salt and baking soda. Stir in the curdled milk mixture until smooth. Scrape the dough out of the bowl onto a floured surface, and shape into a disc. Place the disc into a cast iron skillet.
Bake for 15 minutes in the preheated oven, or until the crust feels firm to the touch.