I have always been under the impression that bikers yield to hikers who yield to horses.
I always try to let the mountain biker pass no matter what the situation, but if push comes to shove then the hiker prevails whether one is going uphill, dowhill, or is uphill or dowhill of the other trail user.
In general, however, it's typically proper for the downhill (i.e. of a lower elevation rather than direction) to yield to the uphill user.
As for wilderness areas, mountain bikers take a huge risk by riding in them. I used to live in Tucson, and I know a # of mountain bikerswho have ridden in the Pusch Ridge Wilderness, and gotten caught. They had padlocks put around their tires so they couldn't ride at all, and then they had to hike down some rough trails to get back to their cars in cycling shoes. In other words: not good. They had to go to the Sabino Ranger Station to get the padlocks off.
One of the big things with "trail etiquette" is being polite. If you're headed uphill, and some guy on a bike is barrelling downhill then it is almost always easier to step aside and let them pass. However, there's no room for jerks because the hiker is ALWAYS doing a favor to the mountain biker. I've found that most say "thanks" when I let them pass, but there are always a few jerks. I had an entertaining run-in in the Phoenix Mountain Preserve a few years back. It was a very windy day so hearing trail noise was difficult, and some clown on a mountain bike came roaring around a corner and down through a dip. He almost hit me and my dog, and I yelled "watch it!" He didn't like it, and came back to me. He replied "what the ---- are you doing??? Watch where you're going!!!" I replied (angrily) that he needed to learn the rules of the _______ trail, and grow up. He didn't like that, but he liked it even less when I told him we could go call the city parks department for a clarification. He was a foul-mouthed moron, and even more so when I pointed out that it was illegal to ride w/out a helmet. What a moron. This clown had all of the requisite gear: $2K bike, cycling clothes...but no helmet. However, in the Phoenix Mountain Preserve morons hiking w/ unleashed dogs is far more of a problem. I've had more run-ins than I can count w/ people whose unleashed dogs have approached mine w/ their hackles up. That's when I bring up the whole "can't you read the clear language on the trailhead signs? Or do you not read English?' Then, when they get huffy, I bring up the whole "we can call the parks department/police to ask them for a clarification, and BTW aren't you carrying bags to clean up after your dog" business.