The downhill rider is least able to do either one due to momentum and gravity pulling them down the hill. The downhill rider is going to take longer to stop than the uphill rider, even if all else is equal and they are moving at the same speed initially.
No, it's not your fault -- the other rider was obviously not taking the proper care to make sure he could stop in time for any obstacles or riders he might meet on the way down. I'm not trying to excuse reckless riding -- if I've failed to make it clear earlier I'll say it again. The original question I resonded to simply asked "whose right of way is it?" and my arguments have been based on the assumption (yeah, yeah -- I know) that both riders are riding responsibly -- if someone is riding like a maniac then all bets are off. My argument is that in the case where both riders can see each other and both could stop in time, the uphill rider should give the right of way if he can.
Well, where do you expect the downhill rider to go? Seems like he would have the same problem (oncoming rider, trail blocked) but fewer evasive options since some of the low-speed manuevers are N/A when going down the hill at speed, and he'll be at a disadvantage in stopping or turning due to momentum and gravity. In fact, in your example above the downhill rider would be at a disadvantage even if you both were going the same slow 5 MPH because it would take him longer to stop. Again, all of this presupposes that the downhill rider is not over-riding the trail and that he can safely stop when he sees something in the trail.

Look, obviously the most important thing is that all riders watch where they're going and make sure they don't become a hazard to others. As long as riders do that, then the question of who has the right-of-way is pretty much moot, isn't it?

If the downhill rider has not taken his "disadvantage" of momentum and speed into account, and reduced his speed enough that he could stop if he had to, then by definition, he is not riding within his safe speed. And that's not the uphill rider's problem. If he has taken it into account, and is able to stop safely, it is very easy to start again... going downhill.

For the uphill rider, depending upon the trail and the rider's technical skills, it may not be possible to start after stopping to let the downhill rider pass. He gets to walk until he makes it past the parts he cannot start on.

I think this is why the majority of all-around riders will at least give lip service to the "uphill has priority" guideline: because of who has the harder time starting up again. However, I agree that in reality, it's usually a case of who notices whom first. I try my best to literally stop, pick up my bike and get off the singletrack for people on foot (or on horseback) as well, but they are so used to being expected to get out of the way that they generally have jumped off the trail before I can even come to a stop.

Protect the earth but dont ruin it for the masses.... Tracks must be left but they must be maintained... If you want to take downhill sections at speed with a locked rear wheel.....goto a actual downhill course! Its been setup to take the abuse of riders 100 times better than the majority of people on this board.

cheers

Ok here's my 2 cents worth...

I ride in a large ravine in a city of 2-1/2 million people.

Im fortunate that I got into mountainbiking in the first place.
I ride with a group every week, several members of this group are awsome xc riders, (not that the free riders are any slouches in their abilities either, dont get me wrong here...)
I really appreciate their abilities, as well as thier trail ettiquette!

We ride at a quick pace, do out best to call around corners, pass with care, warn others that there is "X" number still comming, or we are the last in our group.
Hopefully this behavior pattern will rub off on some riders that are just starting out, like me last year, and keep our sport in the good books.

I try to do my best in not skidding around corners, and feel bad when i do.

I DO NOT want to destroy the awesome trails that are a 15 minute ride from my house, for fear that a bunch of "radical reactionary conservatists" starting to string up fishing line line across the trail like they do in Brittish Columbia!

Keep the rubber side down!

It isn't as bad here as in the states. At least our provincial / municipal and rider clubs work together to find the best places for trails. Our govn't works with the riders to create a happy atmosphere. But yeah the north shore is like ridig through a war zone. Gotta keep you head screwed on right on you may loose it.