This is a book I bought when I decided to trim my own horses, and it has a lot of good info:
I keep both my trail horses barefoot and only boot (front feet) if we get into really rocky terrain. They do GREAT and are sound and happy.
I rode the other day with about 10 other people and was sort of shocked that only me and my friend had our horses barefoot. Every other horse had shoes. Sometimes I forget that the rest of the world is still big into shoeing!
Also, trimming ANY sole that isn't flaking out on it's own is the "kiss of death" for a horse being sound barefoot in the rocks. I always leave any sole that is there, and even if I find some that needs to come out, I only help it out rather reluctantly. That is really the biggest thing I think with trimming to go barefoot, is to never go below the sole and never trim the sole.
The way I look at it is, a serious trail horse needs shoes probably more than any other horse, because they are expected to ride for hours over mixed terrain and rocks. If they can go barefoot, then an arena horse should be able to go barefoot.
And it's more than just "going barefoot." To me, the most important thing is my horse's health and soundness. I honestly feel barefoot is healthiest for the horse's feet and that a lot of soundness issues are caused by poor shoeing.