Combine boiling water and sun-dried tomatoes in a bowl; let stand for 15 minutes. Drain and chop.
Preheat oven to 425°.
Lightly spoon flours into dry measuring cups; level with a knife. Combine all-purpose flour and next 7 ingredients (all-purpose flour through pepper) in a bowl; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. (Flour mixture and butter can also be combined in a food processor; pulse until mixture resembles coarse meal.) Add tomatoes and buttermilk; stir just until moist.
Turn dough out onto a heavily floured surface; knead lightly 5 times. Roll dough to a 1/2-inch thickness; cut with a 2 1/2-inch biscuit cutter. Place on a baking sheet coated with cooking spray. Bake at 425° for 15 minutes or until golden.