Preheat the oven to 400 degrees. Wrap garlic in aluminum foil. Place garlic on rack in oven, and roast until very tender, about 1 hour. Remove from oven, and let cool in foil.
Cut off tops of garlic heads with a serrated knife. Squeeze garlic pulp into a small saucepan. Place over medium-low heat, and whisk in vinegar. Rewhisk, if necessary, before serving. Finely chop rosemary, and add to saucepan. Slowly whisk in olive oil, salt, and pepper, and turn off heat; keep dressing warm until ready to use.