I was watching "How It's Made" the other night, and they featured a segment on full-circle disc brakes. Part of the segment even has a 911 running in the Motorola Cup with them. I'd never heard of these before.
While I can understand why they are not used on most production cars (I'd imagine they are expensive and hard to work on), I don't get why they wouldn't be used in racing and on expensive, high performance vehicles.
More info here:
http://www.brakingtechnologies.com/PDF/SAEPaper.pdf