In response to Andy K's post; Zinn's estimate (which I might have remembered wrongly) was for the frameset weight only. Comparing weights is tricky; there is no guarantee that disc and non-disc models from any one manufacturer are similarly optimised.
If you start with an expedition bike it already has stiff forks etc on it so the weight penalty of adding disc brake fittings to the frameset is likely to be small. [Even so it might be about the same as riding an round with a 6" adjustable spanner strapped to the forks and another to the frame , with much of the extra metal placed to further stiffen parts that just makes the ride less comfortable.]
If you start with a light tourer with a steel frame and fork, the penalty of adding discs is somewhat larger; basically you are likely to end up with a fork that is more akin to an expedition fork (in weight and stiffness, if not outright strength per se) whether you like it or not.
The vexed subject of wheels has not been touched on, but (if built with rims and spokes of the same weight) disc brake wheels are likely to be relatively weaker than rim brake wheels. This is for two reasons
1) the service loads in a disc brake wheel are higher (a lot higher in some places) than in rim brake wheels, and
2) disc brake front wheels are dished (by an amount that is comparable to a rear wheel).
Similarly the loads in the LH bearings are absurdly high in disc brake hubs; really disc brake hubs ought to have bearings that are about 3x stronger in the left side than in the right; that so many disc hubs use bearings that are little different to other hubs just means they have eaten into (and often beyond) whatever safety margin there might have been in the design.
If you were to say that the weight penalty of adding disc brakes to a touring bike with a steel frameset varied from about 0.5kg to 1 kg I would find that a reasonable estimate (about the same as fitting SA hub brakes, which are arguably better....
) . In the grand scheme of things this weight increase isn't the end of the world, but two things are likely to stick in one's craw somewhat;
a) you are likely to be asked to pay more for the privilege of lugging this extra weight around the countryside and
b) more importantly the ride quality suffers with disc brakes fitted, because a good deal of the extra weight goes into stiffening parts of the frame and fork at the expense of comfort.
Right now I'm glad that we have the choice to make between braking systems; doubtless various ones will suit some folk better than others. But in any event it is unlikely that the choice in good quality parts will be maintained in all systems and this is likely to affect touring bikes most adversely; they tend not to have parts specially made for them, and are commonly built using parts that are really meant for something else but just accidentally work OK on a touring bike. A good example is the disc brake itself; if it were only ever fitted to touring bikes, no-one would have ever bothered to make them.