I liked this article from Bike mag so much I am posting it as my blog with minor edit:
...take, for example, the question of how to ride past another rider who is going the opposite direction down the same trail. While old-school guideline dictates that climbers have priority, this general rule of thumb can be taken to ridiculous extremes, such as climbers insisting on the entire trail when it's wide enough to pass. This blind rule following invariably leads to the interruption of a fun descent for no good reason- apart from thoughtless obedience to MTBs traditional rules.
...in the case of climbing versus descending priority one could argue the descender should have priority since they are going at a higher speed, are having more fun and have marginally less control of their mountain bike than the climber, who can usually put a foot down and step out of the way. In that case. the descender enjoys the Flow, and the descender just restarts and continues.
I know plenty of people already on this program when climbing some of our trails. I guess the best bet is to make eye contact and enjoy the ride.