I pull off, talk to the horse and if it's a little freaky I'll take off my helmet too. To pass a horse and rider I let them know I'm coming up, give them a chance to get off the trail and offer to walk past if they want me to. I don't want to be responsible for a horse, rider or both getting hurt and it's only a minor inconvienence. Sometimes a rider asks if the horse can check me out and I'm fine with that too.

Nantanhalla (sp?) also has alternate days for horse/mtb use. They have horses on one trail while mtb use the other so both can use trails the same day and still be separate.