The hubs would be heavier, but the caliper weight and pads is close or equal and in some cases lighter to many rim brakes with pads, esp the latest model years. Check it out for yourself:
http://weightweenies.starbike.com/li...type=rimbrakes
http://weightweenies.starbike.com/li...ype=discbrakes
I took my neighbors Schwinn World DBX (Avid BB7 and carbon fork) for a spin and you can do one finger braking at just about any speed. The brake modulation and feel is fantastic. With the long hills he commutes on, I can see why he loves disc brakes and bought that particular bike. Replacing rotors should be preferable to replacing rims. He was telling me how easy is was to adjust and replace the brake pads "by hand"...lol. He has the pads pretty close to the rotor but they are not touching...he says the manual says backoff 2-3 clicks on the adjustment knob for both the inner and outer brake pad setting, but he sets them as close as possible without the pads rubbing/dragging...so I guess it's 2 clicks or less. He was telling me there are different caliper adapter sizes, so he could install up to a 203mm rotor on the front if he wanted to. The stock 160mm rotors (f/r) seem to be good enough, although I would probably like to see a 185mm rotor on the front for a better overall brake bias/balance.
I expect disc brakes would show up first on touring bikes (disc brakes would be an advantage on a fully loaded tourer) and a limited model/number for road bikes. It's probably more tradition and total weight considerations that would hamper the wide spread implementation and acceptance of disc brakes on road bikes. Doubt you will see any disc brakes in the TdF unless someone goes lighter in the frame, rims, or fork to compensate for the additional weight of the hubs or can get the bike weight down to the minimum 6.8 kilograms bike weight...and you'll probably see disc brakes only on the mountain stages. The advantage would be minimal/negligible at best and the arguement would be the lower center of gravity of the brake placement. And there is the aero question too. The Mtn bikers are into juicys now and you don't see many using rim brakes anymore.
In any case, the consumer and demand will determine whether or not we will see a wider selection of disc brake equipped road bikes.