I love the idea that every mt. biker would have to have a course in trail etiquette before using the trails. We are not the group that causes the most trail damage, but we are the group that draws the most ire because we (as a group) continually startle other trail users and cause them to scramble and jump. We have the most to lose, and our relationships with other trail users are tenuous. Adding motorized mt. bikes isn't going to help either.
So - should we have another tax and fee? Not at all. But as more and more uninformed riders take to the trail, some with unnatural wattage at their disposal, maybe a certificate of non douchebaggery would be a good thing. Classes could be taught by volunteers.
But then what? Database management and enforcement is a whole other thing - and not something I would like to see. I know - turnstiles at every trail entrance, activated by an electronic sensor on your "license" and a retina scan for authentication!
Additional thought - if similar licenses were required by all user groups, and they were taught that bikes must yield to all, imagine the self righteousness if we actually tried to ride past a group of hikers rather than stopping to wait for them to walk/jog by. That would be especially fun if they are going the same direction we are, on a narrow single track with little opportunity to safely pass. My experience is that pedestrians step aside for bikes all the time. It's just easier that way. We shouldn't expect them to, and we should use awareness bells and be extra courteous when they do step aside, but imagine if they didn't!