I just built up a new frame and discovered that, due to slightly different geometry the rear disc caliper can't be adjusted so that it doesn't rub. Basically I've got them all the way over to one side and it's still not enough. Can I fix this without getting new brake calipers? They're Shimano BR-M406 hydraulic caliper BTW.
Ӎσᶎ's answer was the method I went with. I just got a washer from another hub (I didn't want to use a plain old steel washer from the hardware shop because I was concerned it might be too soft, though that might not be the case) and put it on the axle next to the cone. I did that In January and it's done about 4000k without any problems since then.
You can buy shims to put between the disc and hub. Here's one example: http://www.jensonusa.com/Syntace-Disc-Rotor-Shims
It's really that simple.