I am not a fan of wearing earphones while riding, and especially those on the trail with me, wearing them.
I am a cyclist first. Nothing worse that coming up on someone and yelling at them when you are passing, and they don't hear you.
I haven't come up on someone on a wheel carving back and forth (like the roller bladers). But I know I have almost hit many a joggers that are oblivious.
Wearing a full face motocross helmet, then putting earbuds in, I can't imagine you could hear anything. I know with 30MPH wheels, you don't expect to get passed, but I'm sure it happens. I wouldn't want to be the one taking a group of cyclist down and giving wheel riders a bad rep for not sharing the road.
(OK, you can kick me of this site now)