Don't know about nVidia but for ATi, the o/cing issue is anything but clear cut. I mean, CCC allows you to o/c your cards (within certain limits, but how can one know if the limit was breached?), and i doubt manufacturers would come up with warranty conditions that contradict ATi's official support.
I know on the nVidia side of things, you have to accept a EULA before overclocking with the software that they provide. But, ... you're right, it's not clear exactly which cards you can overclock and not void the warranty until you read very carefully. AMD doesn't need to be the arbiter of what cards can be returned to their manufacturers. If Company A wants to warranty their card with overclocks and Company B does not, then they can each do their own thing. People that bought Company A's cards are free to overclock and if their card melts, they can send it back to Company A. If someone with Company B's card overclocked it and it failed, they can send it back and hope they'll replace it, but that is fraud.