Recently our local group went out to play, and one of our players got their tooth chipped in half. We play fairly regularly (almost every weekend) with 10-20 people so an incident was inevitable after a year and a half of fairly regular play.
We use standard FPS limits 400 for AEG's and 550 for bolt's along with the z87.1 eyewear. It was a stock Echo 1 mp5 that did the dirty work to my friends tooth at about 75 feet. It bairly hit the nerve of the tooth so there is a chance they will have to do a root canal. I can safely say that a majority of our players will be using mouth protection in the future. 1-3$ for a mouth guard vs. several hundreds to fix a tooth.
It is rare, but it can happen. I want to suggest wearing some sort of balaclava or mouth protection, while it isn't life threatening might as well play it safe eh?