Heat the oil in a heavy medium skillet over medium heat. Add the pancetta and saute until crisp and golden, about 5 minutes. Using a slotted spoon, transfer the pancetta to a paper towel-lined plate to drain the excess oil.
Stir the buttermilk, sour cream, and garlic in a medium bowl to blend; mix 2/3 of the cheese. Season the dressing with salt and pepper, to taste.
(The dressing can be made up to 2 days ahead. Cover and refrigerate. Stir to blend before using, adding more milk to thin the sauce, if necessary.)
Arrange each of 6 lettuce wedges on 6 plates. Spoon the dressing over the lettuce. Sprinkle with the pancetta and remaining cheese and serve.