Building road bikes which are lighter than the rules allow has been possible for quite a while now, and like many similar areas of engineering, racing drives the breed. Integrating the extra weight of disc brakes is therefore not really relevant excepting for how it might affect steering geometry, easily overcome by design. I'm thinking it's been more of an attitude-inertia question than a design one, especially when you consider the quantity of creative thinking devoted to reducing the aerodynamic disadvantage of rim brakes over the years. For sheer strength and ease of modulation, to me discs are obviously the better choice for braking. The changeover is gathering momentum, and I suspect that by this time next year it'll be a lot easier to find a production road bike or a replacement disc fork for a road bike (as I've recently anguished over) than it is today.
Edited to add a comment for the near-simultaneous above post (I weigh my words carefully, and it takes a long time to compose): That Colnago is dead sexy. Oh, yeah, I would.
Last edited by SuperDave; 09-13-12 at 09:22 PM.