Here's some video of my first lap at Corsicana
http://www.youtube.com/watch?v=dY1Vp3SOhw8 Go to minute 13:00 when I come up on a couple guys in another class. You'll see proper etiquette by the guys I'm coming up on. I only have to rev once or twice and they hear me and move over.
What I've come to understand is not everyone can hear me. Maybe because I'm on a 2 stroke. I learned that when I came up on guy I know at Legacy Farms in the woods and I was revving and yelling and it took longer than I would have liked for him to realize I was there. Afterward he said he never heard me. I certainly believe him because he's a good guy and knows I usually catch at some point during the race. That incident helped me realize that not every guy who doesn't immediately move over is doing it on purpose.
I've had a race or two where I passed the entire class in front of me and for the most part even the guys leading their own race moved over pretty quick. I've had a few notable cases where a guy just wouldn't move over and in those cases I eventually have to force the issue, but I'd never try to take someone out. It's not like I want to crash either.
I'm sure this is probably a bigger issue in the amateur/novice race groups because an intermediate class is mixed in with them and a lot of the racers are inexperienced, hyped up and in race mode. There's never going to be a perfect answer when faster riders from one class are coming up on slower riders from another, but if all the new racers starting out are educated and reminded of this issue things should go fairly smooth for the most part.
This is my first year racing cross country so I've sure had to learn the ropes. This year I'm the passer and next year when I move up I'll have to learn how to pull over for faster guys coming through.
That's my 2 cents anyway.