Here's a link to Equine Ink
about bits and the anatomy of the horse's mouth. It's brief, but looks like more will be forthcoming. At the bottom of the article is a link to a USDF article that's more technical--radiographic images of the ways different snaffle bits sit in the horse's mouth.
Once upon a time I saw an article explaining how to determine if your horse was having problems with the bit (as opposed to other things that would cause him to avoid contact). If anybody finds it, you might post the link. It's useful information. In the meantime, I'll continue to look.