There are good points made by many people.
Hopefully everyone can agree that whenever a person (rider or hiker) stops, you should move aside to allow others to pass without them having to ask.
Another thing to consider for some of you, is that the Black Diamond Down Hill, is in fact a hiking trail called The Good Samaritan. This trail pre-dates Mnt Biking. Watch for people, dogs, and slower riders over the hills.