Mongoose has a somewhat elegant, neat looking aluminum dropout on the rear of their cyclocross frame which allows for both a disc brake (it's coming, oh yes, mark my words) and a rack + fender mount. If only I could read Hungarian.
Many companies have tried this with varying levels of slick-ness. For example, here's Voodoo's attempt at making the rear dropout that is all things to all people:
My take on putting a disc brake braze-on on a non mountain bike is that you want to localize the stresses of braking in the forged, stamped or CNC'd dropout part instead of transfering them onto the slender, flexible seat stays. Niner places theirs there, but then has to beef up the joint with a welded-in gusset.
I particularly like the Mongoose approach because it keeps the disc brake out of the way of the fender or rack, which prevents this giant kluge: