To the best of my knowledge the dogs used for human consumption in China are specially bred for the purpose; seem to remember they're hairless and look a bit like a goat. Had some in a little restaurant in Beijing once - the meat is not dissimilar to beef but a lot tougher.
That's an interesting observation about dogs being carnivores/scavengers and a possible taboo existing on eating them. I suppose you could say pigs are scavengers as well in so much as they'll eat pretty much anything - hence the 'unclean' image for some?). Trying to think what other carnivores are eaten by humans. Most (all?) fish I suppose could be regarded as carnivorous but apart from fish not that many - crocodile, alligator and snake spring to mind but can't think of anything else.
Would imagine the Neolithic dog still pretty much resembled the wolf and m'be just didn't taste very good. For a similar reason lamb and mutton in the Far East is still hardly eaten, not because it doesn't taste good but because of the way it smells! Same applies to cheese (in a part of the world where the cuisine has no dairy tradition to speak of).
