regarding dogs "Is it temperament you breed for or is it training that results in their seemingly easy malleability?"
I'd say it's both. Most of us don't breed dogs that are obviously spooky or agressive.
But mostly it is just experience. Many people think sled dogs live in isolation and aren't socialized, but the opposite is true. They live in large, well adjusted packs of dogs. Yes - well adjusted. They have to be, or it would be non-stop mayhem. Our dogs are frequently let loose to run around, play, and interact.
Even when they are on their chains, they can play and interact with dogs near them, and are completely aware of everything going on in the lot. If a dog steps over its chain, every single dog in the lot will pitch a fit. Even those on the opposite side of the kennel are completely aware of what is happening with their pack-mate.
We go to races where they interact with sometimes literally thousands of strange dogs, and they have to know how to get along there too. Compared to the average pet dog, who has to adjust to only one or a few pack members, and maybe learns to cooperate with a handful of others if they go to a dog park, a sled dog is much more well adapted to social interactions with other dogs.
The same is true when it comes to being handled by strangers or traveling or coping with strange new situations. This is the norm for a sled dog. They take their cues off each other and from me, and they learn to take everything in stride.
Another thing that helps these dogs adjust to things easily probagbly has to do with how they are perceived and treated by their mushers. I take my responsibilities for my dogs very seriously, and I provide them the best care available. I love them, with a bond that I have never felt with any pet dog before. But I don't consider my dogs to be my 'babies' or my 'children'. They are team mates. I don't coddle them. These guys? Come on! They are tough, determined, stoic sled dogs. To treat them as pseudo-infants is insulting. I know they can deal with new experiences, and my confidence that they will be fine means that they generally are. If I were to soothe and encourage and fuss over them each time something potentially scary happened, it would just confirm to them that something scary was happening. Being matter of fact and trusting them to be fine tells them that they are fine. Not to say I don't spend plenty of time babying my dogs if they are sick or hurt or something truly scary has happened. See my January 15, 2010 post on my website for more soap box on this subject...