Luckily, I live close to Elliot Bay Bicycles, home of Davidson Bicycles. If you've got local frame builder, that would be the establishment to hit. They'll probably roll the dent out, then fill whatever is left in with brazing. If the color of paint is common and easy to match, they might be able to touch that up. But if it's not, this would also necessitate a repaint. Too bad about the dent, pretty bike.