Yeah, this does kinda smell like trouble. Oh well, here goes...

Of course decent people should try to make room for each other. Luckily it seems that everyone sharing trails in my area is of the same mindset as myself and we always slow down for walker/hikers and get all the way to the edge of the trail, as do the walker/hikers for us. Usually, as Samburger said, it's also accompanied by a friendly greeting of some sort. However, many road bikers in my area are just plain self-centered jackasses. I swear, if you observe them(I've ridden along with some of them), it's like they get their jollies on inconveniencing others to prove some sort of a point that only they seem to know what it is or think it needs proving. As both a MTBer and a road biker, I really don't understand why there's such a difference, but it's definitely there. If your trail walkers act like the road bikers here, I say call out in an assertive voice "on your right/left", get as far to your side of the trail as possible, and maintain a safe speed but returning whatever sentiment they're giving you. If they seem like perfectly nice, cool, people and they're giving you room, slow way down and give a greeting. If they're being jackasses about it, be a polite but assertive jackass back. Do be extra careful around the animals, though, as you should anytime passing on the trails. It's not their fault if their owner is a jackass.