I would not consider myself an "NH'er", but I will respond anyway. Yes, a horse does begin its relationship with the prey/predator barrier. However, that is just what it is, a barrier. A barrier can be broken, and once it is it is no longer that barrier, but rather a pecking order issue. I have recently been working with one horse that was extremely fearful of people. After several session, the horse began relaxing and approaching me. He was happy to see me. The first thing he did when he realized I wasn't going to eat him was test me. He went immediately from fear of the human to trying to push on my personal space. I set the boundary, and he was fine with me being the herd leader. So yes, we are naturally predators, but that can be overcome, and then the next issue is what we address, where we stand in the herd. I have seen my horse protect my old black lab from other dogs on more than one occasion. A prey animal taking on a predator to save another predator? I guess no one ever showed him the rule book! That horse and that dog went on hours of rides together, they formed a relationship.
Then there is the ever controversial question of smacking a horse. This could be taken in several ways. Like CJ stated, it could be a reaction. If I have one of my horses bite me, then I obviously haven't shown myself to have the leadership skills to let the horse know its not acceptable. I do seem to remember a post about PP smacking spirithorses's horse on the butt to get him to move out and increase drive. I have no problem with this at all. But this does go to show that you can't say that all contact like that is bad. Once you have passed through the prey predator barrier, it is sometimes necessary to establish your place as herd leader "biting" that horse on the butt.
There is no one way that is perfect. A good leader is adaptable, which is going to transfer into doing what the horse needs even if it does deviate from the original plan.